THE
INSTITUTIONS
OF
PHYSIOLOGY. SECT. I.
OF THE LIVING HUMAN BODY IN GENERAL.

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1. In the living human body, regarded as a peculiar
organisation, there are three objects of consideration.^*

The materials of its subsistence, afforded by the
fluids;

The structure of the solids, containing the fluids;

Lastly, and principally, the vital powers, by which
the solids are enabled to receive the influence of the
[Seite 2] fluids – to propel the fluids – and perform various other
motions; and which, as they, in a certain sense, con-
stitute the essence of the living machine in general, so
also are of very different orders – some being common
to animals and vegetables – some peculiar to animals
and intimately connected with the mental faculties.

2. But these three, although really distinct, and
therefore distinctly considered by us, are so closely
related in the living system, (the phenomena, conditions,
and laws of whose functions, in the healthy state, are
the object of physiology) that no one can be contem-
plated but in its relation to the rest.

For the materials of the body, although originally
fluid, are naturally disposed to become solid; and, on
the other hand, the solids, besides having been formed
from the fluids, abound, however dry they may appear,
in various kinds of fluid constituents, both liquid and
aëriform: lastly, it may probably be affirmed, that no
fibril, during life, is destitute of vital power.

3. We shall now examine each of these separately;
and first, the materials afforded by the fluids, which
form both the fundamental and most considerable por-
tion of our bodies.^*

SECT. II.
OF THE FLUIDS IN GENERAL, AND PARTICULARLY
OF THE BLOOD.

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4. The fluids of the body may be conveniently re-
duced to three classes.

A. The crude; viz. the chyle, contained in the primae
viae and destined to become blood; and matters ab-
sorbed on the surface and destined to be conveyed to
the chyle.

B. The blood itself.

C. Those secreted from the blood, whether inert and
excrementitious, like the urine; or intended for certain
purposes in the economy: the latter may be perma-
nently liquid, as the bile; or disposed to solidity, as
the osseous and other plastic juices.

5. Of the first and third of these classes we shall
hereafter speak, in treating of chylification, secretion,
and the other functions to which each fluid appertains.
At present our attention shall be devoted to the blood^* –
the chief and primary fluid – the vehicle of those suc-
cessions of oxygenous and carbonaceous particles,
which cease with life only – the nourisher of the frame
– the source of almost every fluid – that into which the
crude fluid is converted and from which all the secre-
tions are derived – and which, with the exception of
some exsangueous parts, as the epidermis, the arach-
noid, the amnion, &c. the vitreous substance of the
[Seite 4] teeth, the body of the crystalline lens, &c. is univer-
sally diffused through the system, in various propor-
tions, indeed, according to the various natures of the
similar parts, to use the language of the ancients,^*
v.c. abundantly in the muscles and glands, sparingly
in the tendons and cartilages.^† (A)

6. The blood is a fluid sui generis, of a well known
colour and peculiar odour; its taste is rather saline
and nauseous; its temperature about 96° of Fahrenheit;
it is glutinous to the touch; its specific gravity, though
different in different individuals, may be generally esti-
mated as 1050, water being 1000; when fresh drawn,
and received into a vessel, it exhibits the following
appearances.^‡

7. At first, especially while still warm, it emits a
vapour which has of late been denominated an animal
gas and shewn to consist of hydrogen and carbon, sus-
pended by caloric.^§ This, if collected, forms drops
[Seite 5] resembling dew, of a watery nature, but affording a
nidorous smell, which is most remarkable in the blood
of carnivorous animals, peculiar and truly animal.
Much of this watery liquor still remains united with
the other parts of the blood. (B)

8. In the mean time the blood, when its temperature
has fallen to about 78°, begins to separate into two
portions. A coagulum is formed, from the surface of
which exudes, as it were, a fluid of a yellowish slightly
red colour, denominated serum: the more abundantly
this exudes, the greater is the contraction of the glu-
tinous coagulum, which has received the appellations
of crassamentum; and, from some resemblance to the
liver in colour and texture, of hepar sanguineum; of
placenta; and, from the circumstance of its being sur-
rounded by the serum, of insula. (C)

9. The crassamentum may, by agitation or repeated
ablution, be easily separated into two constituent
parts – the cruor, which imparted to the blood its
purple colour, and the lymph, which on washing is for-
saken by the cruor, and called, from its greater solidity,
the basis of the crassamentum. The stronger affinity
of the cruor for the lymph than for the serum, is proved
by the necessity of violence to effect their disunion.
By the removal of the cruor the lymph becomes gra-
dually paler, till it is at length merely a white tenacious
coagulum. (D)

10. Besides the watery fluid first mentioned, these
are the three constituents of the blood, viz. the serum,
the cruor, and the lymph: we shall presently treat of
each more particularly. These, however, while recent,
and in possession of their native heat, are intimately
[Seite 6] mixed, and form an equable, homogeneous fluid. Their
relative proportion is astonishingly diversified, accord-
ing to age, temperament, diet, and similar circum-
stances which constitute the peculiar health of each
individual.

11. The serum is a peculiar fluid, the chief cause of
the viscidity of the blood, and easily separable by art
into different constituent principles. If subjected to a
temperature of 150° Fahr. a portion is converted into a
white scissile substance, resembling boiled albumen:
the rest exhibits, besides the watery fluid so often men-
tioned, a turbid fluid of a gelatinous, or rather mucous,^*
nature, which on cooling appears a tremulous coagu-
lum. The serum is remarkable for the quantity of soda
(mineral alkali) which it contains. (E)

12. The cruor has many peculiarities, in regard to
both the colour and the figure of its particles. It con-
sists of globules, which in recent blood are of a constant
form and size, and said to be 1/3300 of an inch in dia-
meter. Their form, indeed, has been a subject of dis-
pute; but I am disposed to consider it as much more
simple than some writers of great celebrity have ima-
gined. I have always found it globular, and could
never discover the lenticular shape which some have
asserted that they remarked.

It has been likewise advanced, that the globules
change their form, while passing through a vessel of
very small capacity – that, from being spherical, they
become oval; and, when they have emerged into a
[Seite 7] vessel of larger area, that they again resume their glo-
bular shape.^* This, although I would by no means
deny it, I cannot conceive to occur during the tranquil
and healthy motion of the blood, but should refer it to
a spasm of the small vessels.

Their globular figure can be seen in a living animal
only, or in blood very recently drawn: for they are
soon unobservable, becoming a shapeless mass which
resembles serum in every circumstance excepting
colour.

13. Their colour is red, and from it is derived the
colour of the blood. In intensity it varies infinitely;
paler in animals which have been poorly nourished or
have suffered from haemorrhage; more florid, when
oxygenised^† (rendered arterial, to use the common
phrase) by exposure either to atmospheric air, or, more
especially, to oxygen; darker when carbonised, (in
common language, rendered venous) by exposure to
carbonic acid gas, or to hydrogen.^‡ The redness is
[Seite 8] most probably to be ascribed to the oxide of iron,^* the
quantity of which, however, is so minute, that it has
been most variously estimated. (F)

14. The last constituent principle of the blood to be
noticed, is the plastic lymph, formerly confounded with
the serum. This has been called the basis of the cras-
samentum, the glutinous part, the fibre or fibrous matter
of the blood, and, like the caseous part of milk and the
gluten of vegetables, been discovered by late analysis
to abound in carbon and azote. (G)

15. It is properly denominated plastic, because it
affords the chief materials from which the similar
parts, especially the muscles, are immediately pro-
duced; nourishes the body throughout life; repairs
wounds and fractures in an extraordinary manner; fills
up the areae of large blood vessels when divided; and
forms those concretions which accompany inflamma-
tions,^† and that remarkable deciduous membrane found
in the recently impregnated uterus for the attachment
of the ovum.

16. Thus much have we said, respecting the consti-
tuent parts and nature of the blood, the most important
fluid of the animal machine, – a fluid, which excites the
heart to contraction; which distributes oxygen to every
part, and conveys the carbon to the excretory vessels,
giving rise, by this change, to animal heat; which sup-
[Seite 9] plies the materials of the solids originally, and after-
wards their nourishment; and from which all the other
fluids, with the exception of the crude (4.), are secreted
and derived. Of the multifarious importance of the
blood, we shall speak particularly hereafter.

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SECT. III.
OF THE SOLIDS IN GENERAL, AND OF THE MUCOUS
WEB IN PARTICULAR.

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17. The solids^* are derived from the fluids. In the
first rudiments of the gelatinous embryo, they gradually
commence in their respective situations, and differ in-
finitely in their degrees^† of cohesion, from the soft and
almost pulpy medullary matter of the brain, to the
vitreous substance of the corona of the teeth.

18. Besides the gelatinous (11) and glutinous (15)
parts of the solids, earth enters more or less into their
composition, and is principally lime united with phos-
phoric acid. The bones possess this in the greatest
abundance, particularly in advanced age, whereas in
childhood the gelatinous matter abounds.

19. With respect to texture, a great part of the
solids consist of fibres more or less parallel. This may
be observed in the bones, especially of foetuses,^‡ in the
[Seite 14] muscles, tendons, ligaments, aponeuroses, and in cer-
tain membranes, as the dura mater, &c.

20. In other parts no fibres can be discovered, but
the texture is peculiar, has been called parenchyma
from the time of Erasistratus, and differs in different
viscera, especially the secreting, – of one kind in the
liver, of another in the kidneys.

21. But in all the structures, whether fibrous or paren-
chymatous, there is interwoven a general mucous web,^*
commonly styled cellular, but improperly, because it
rather is continuous, equal, tenacious, ductile, sub-
pellucid, and glutinous.^† By handling, it is easily
converted into a cellular and vesicular membrane, and
demands a place among the most important and re-
markable constituents of the body. (A)

22. For, in the first place, many solid parts, v.c.
most membranes and cartilages, may by long continued
maceration be resolved into it alone. With some it is
so intimately united, as to afford a receptacle and sup-
port for other constituents: v.c. the hardest bones
consisted at first of cartilage, that was originally con-
densed mucous membrane, but has since become dis-
tended by the effusion of bony matter into its substance,
which is rendered more lax and cellular. In fact, it is
universally present in the solids, if we except the epi-
dermis, nails, hairs, and the vitreous exterior of the
corona of the teeth, in which I have never been able to
discover it by employing the strongest acid.

23. To the muscles and membranes especially it
[Seite 15] serves for separation from other parts; to the vessels
and nerves for support; and to every part it acts as
the common medium of connection.

24. From these facts, two inferences may be drawn.
First: That this membrane is so fundamental a consti-
tuent of our structure, that, were every other part re-
moved, the body would still retain its form.

Secondly: That it forms a connection between all
parts of the system, however different from each other
in nature or remote in situation: – a circumstance
worthy of attention, as putting an end to the verbal
disputes respecting the continuation of membranes, and
affording an explanation of many morbid phenomena.

25. As most of the solids owe their existence to this
membrane, so again its origin is derived from the lymph
of the blood. I have found the lymph changed into
this membrane, when transuded on the surface of in-
flamed lungs, and, by forming false membranes, it
afterwards unites these organs to the pleura.

26. We shall now consider some varieties of this
membrane. In general, it is more delicate, caeteris pa-
ribus, in man than in animals, – a distinguishing prero-
gative, by which our sense is rendered more delicate,
and our motions and other functions more perfect.^*
Among different individuals, it varies much in laxity
and firmness, according to age, sex, temperament, mode
of life, climate, &c.

Finally, it varies in different parts; – more lax in the
palpebrae and preputium, and behind the fraenum of the
tongue; less so around the ears.

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27. Besides the purposes before mentioned (22,) (23),
it is destined for the reception of several kinds of fluids.
Its chief use in this respect is to receive that serous
halitus which moistens and lubricates every part. This,
when formed by the blood vessels, it imbibes like a
sponge and delivers over to the lymphatics, thus consti-
tuting the grand connection between these two systems
of vessels.

28. In certain parts its office is to contain peculiar
fluids; v.c. in the eye, existing as the vitreous mem-
brane, it contains the vitreous humour: in the bones,
as the medullary membrane (improperly denominated
internal periosteum), the marrow; in soft parts, it is in
great abundance, and contains the rest of the fat, of
which we shall speak hereafter.

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SECT. IV.
OF THE VITAL POWERS IN GENERAL, AND PARTICU-
LARLY OF CONTRACTILITY.

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29. Hitherto we have spoken of the solids as
the constituents of the system; we now shall view them
as endowed with vitality, – capable of receiving the
agency of stimuli, and of performing motions.

30. Although vitality^* is one of those subjects which
are more easily known than defined, and usually indeed
rendered obscure rather than illustrated by an attempt
at definition, its effects are sufficiently manifest and
ascribable to peculiar powers only. The epithet vital
is given to these powers, because on them so much
depend the actions of the whole body during life and
of those parts which for a short time after death pre-
serve their vitality, that they are not referrible to any
qualities merely physical, chemical, or mechanical.

31. The latter qualities, however, are of great im-
portance in our economy. By physical powers, de-
pendent on the density and figure of the humours of the
eye, the rays of light are refracted to the axis; by me-
chanical, the epiglottis is elastic; by chemical affinity,
the changes of respiration are effected. But the perfect
difference of these dead powers from those which we
[Seite 18] are now about to examine, is evident on the slightest
comparison of an organised economy with any inorganic
body, in which these inanimate powers are equally
strong.

32. Indeed the vital powers are most conspicuously
manifested by their resistance and superiority to the
others; v.c. during life, they so strongly oppose the
chemical affinities which induce putrefaction, that Stahl
and his followers referred their notion of life to this an-
tiseptic property;^* they so far exceed the force of
gravity, that, according to the celebrated problem of
Borelli, a dead muscle would be broken asunder by
the very same weight, which, if alive, it could easily
raise; &c.

33. As on the one hand, the vital properties are com-
pletely different from the properties of dead matter, so,
on the other, they must be carefully distinguished from
the mental faculties which will form the subject of the
next chapter: between them, however, there exists an
intimate and various relation, observable in many phe-
nomena, but especially in the diversity of tempera-
ment.

34. The vital energy is the very basis of physiology,
and has therefore been always noticed, though under
different appellations. The titles of impetum faciens,
innate heat, archaeus, vital spirit, brute life, head of
the nervous system, active thinking principle, vital
tonic attraction, have been bestowed upon it by dif-
ferent authors.

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35. Nor has there been less variety in the notions and
definitions to which it has given rise; though in this
one point all have agreed, – that its nature and causes
are most obscure.

36. As to the question so long agitated by physiolo-
gists, – whether the diversity of the phenomena exhibited
in the similar parts of the living solid is to be attri-
buted to modifications only, or to distinct species, of
the vital energy, we think it best to establish distinct
orders of the vital powers, according to the variety of
phenomena by which they are manifested.

37. These phenomena are threefold. – Organic for-
mation and increase; motion in the parts when formed;
sensation from the motion of certain similar parts.

38. The first requisite involved in the name and notion
of an organised body, is a determinate form designed
for certain ends. That species, therefore, of the vital
powers is most general, which produces the genital and
nutritive fluids and prepares them for organic nature.
This species we have denominated the nisus formativus,
since it is the source of all generation, nutrition, and
reproduction, in each organised kingdom.

39. Those vital powers which are manifested by
motion, (37) properly so called, in parts already formed,
may be divided into common and proper. The common
are those belonging to similar parts which are widely
distributed: v.c. contractility to the mucous structure;
irritability to the muscular fibre. The proper are those
possessed only by individual organs whose motions
are peculiar and characteristic.

40. Contractility is as generally distributed as the
mucous structure, which it may be said to animate; and
therefore would perhaps not improperly be called the
[Seite 20] vis cellulosa. It is characterised by a simple and not
very sensible effort of the mucous web to contract and
react upon its contents, especially upon its source of
moisture – the serous vapour, and to propel this into
the lymphatic system.^*

41. Irritability, we mean the irritability of Haller, is
peculiar to the muscles, and may be called the vis mus-
cularis. It is marked by an oscillatory or tremulous
motion, distinguished from the action of simple con-
tractility, by being far more permanent, and by occur-
ring far more easily on the application of any pretty
strong stimulus.^†

42. Such are the common (39) moving vital powers.
But some organs differ from the rest so much in their
structure, motions, and functions, as not to come under
the laws of the common orders of vital powers. We
must, consequently, either reform the characters of these
orders, institute new ones, and extend their limits, or,
till this be done, separate these peculiar motions from
the common orders, and designate them by the name of
vitae propriae.^‡ As examples may be adduced, the
motions of the iris; the erection of the nipple; the
motions of the fimbriae of the Fallopian tubes; the
action of the placenta and of the womb during labour;
[Seite 21] and probably the greater part of the function of se-
cretion.^*

43. So much in regard to the vital powers displayed
by motion (37, 39, 42). We have now to speak of
sensibility, which is peculiar to the nervous medulla
communicating with the sensorium. It bears the title
of vis nervea, and is the cause of perception when irri-
tation is excited in parts to which it is distributed.^†

44. The order which we have followed in enume-
rating the vital powers, (38, 43) is that in which they
successively arise both during our formation and after
birth.

The nisus formativus must take place before we can
ascertain the existence of the new conception.

Then contractility is exerted in the gelatinous sub-
stance of the embryo.

When the muscular fibres are produced, they acquire
irritability.

In those few organs whose motions cannot properly
be referred either to contractility or irritability, there
next exists a vita propria.

Finally, after birth, sensibility is superadded.

45. Similar also is the order, according to which the
vital powers, both common and proper, are distributed
to the organised bodies of each kingdom.^‡

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The formative power must be most universal;
without it indeed organisation cannot be conceived to
exist.

Contractility likewise is common to each kingdom.

Irritability and sensibility, in the sense above ex-
plained, are peculiar to animals.

Lastly, the vita propria is variously observable in
some organs, particularly the generative, both of certain
animals and vegetables.

46. It is scarcely necessary to remark, that most of
these modes of vital energy, though necessarily distin-
guished into orders, are intimately connected; v.c. the
mucous web, forming the basis and seat of contractility
in so many organs, is interwoven also with the irritable
muscular fibres^* and the sensible nerves.

47. Whatever may have been the opinions of physi-
ologists respecting the difference or identity of the vital
powers, it is universally agreed that they exist in the
similar solid parts, as the ancients called them, of which
the organs or dissimilar parts are composed. But it
has been disputed, and particularly of late, whether
vitality is peculiar to the solids or common also to the
fluids; and, the latter being granted, whether or no the
blood alone is so endowed.

48. As to the first question, the whole natural history
of each organic kingdom, as far as it has hitherto been
cultivated, abundantly shows that those living parts,
however delicate, of all known animals and vegetables,
are solid; – a circumstance necessarily implied in their
determinate figure destined for certain uses. For, not
[Seite 23] to speak of entire animals (which, however simple, as
worms, are nevertheless supplied with enveloping mem-
branes) the newly laid egg, though at first sight merely
fluid, on a more careful examination is discovered to
consist of different membranes, of the halones, the cica-
tricula, &c.

Humidity is indeed necessary in the living solid for
the exertion of vitality. But that vitality exists in the
solid, as solid, is proved by the well-known instances
of animalcules and the seeds of plants, in which,
although long dried, the vital principle is so entire, that
they again live and germinate.

49. With respect to the supposed exclusive vitality
of the blood, I candidly confess that no fact has been
adduced in its favour since the time of Harvey, which
might not, I think, be more easily, simply, and naturally
explained on the contrary supposition.

For example, the incorruptibility of the blood during
life, is far more explicable by the perpetual changes
which it undergoes, especially in respiration.

That the blood is the material from which the living
solids are produced, is no stronger an argument of its
vitality, than the formation of nymphaeae and of so
many other remarkable plants would be for the vitality
of water.

It is difficult to comprehend how the coagulation of
the lymph of the blood can demonstrate its vitality.
The organic formation of this lymph in generation, nu-
trition, and reproduction, depends not upon the lymph
itself, as lymph, but upon the action of the nisus for-
mativus (38) upon it.

50. Those who formerly contended^* that the blood
[Seite 24] acquires in the lungs from the air a certain principle to
be universally distributed during circulation, for the
purpose of imparting motion, &c. to the organs, were
right, if they regarded that principle (analogous to the
oxygen of the moderns) as the stimulant of the living
solid; wrong, if they regarded it as vitality itself.

51. For it is on all hands agreed, that no motion oc-
curs but upon the action of stimuli, to receive which
action the vital powers are naturally adapted and
intended.

52. These stimuli,^* however multifarious, are conve-
niently reduced to three classes; – chemical, mechanical,
and mental. For the present, we shall say nothing of
their various modes of action, – in some instances
direct, – in others indirect, by sympathy and sensorial
reaction. It is sufficient at present to cite a few ex-
amples of functions, to which each class of stimuli
conspires: such is the increased secretion of tears,
saliva, bile, &c. and the venereal turgescence of the
genitals.

53. If the nature of stimuli is infinitely various, no
less so are their effects, according to their nature, inten-
sity, or continued and repeated application to the living
solid. Hence they are generally divided into exciting
and depressing.

54. The power of certain stimuli in increasing the
[Seite 25] effects of others, is very remarkable: v.c. the power of
caloric, upon which probably national temperament
chiefly depends.^* That of joy, a most energetic mental
stimulus, is similar.^† Likewise perhaps that of
oxygen, (50) by whose chemical stimulus the vital
powers, particularly irritability, are greatly excited,
and more disposed to react, upon the impulse of other
stimuli.

55. Not less considerable than the variety of stimuli,
is that more minute discrepancy of the different organs,
and of the same organs in different individuals, accord-
ing to age, sex, temperament, idiosyncrasy, habit, mode
of life, &c., to which are owing the diversified effects of
the same stimuli upon different organs,^‡ and even upon
the same in different individuals, and upon which
depends what the English have lately termed specific
irritability.^§

56. Lastly, the influence of stimuli by means of sym-
pathy, is very extraordinary: by its means, if one part
is excited, another, frequently very remote, consents in
feeling, motion, or some peculiar function.^ǁ

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The primary and most extensive cause of sympathy
must be referred to the nerves,^* and indeed chiefly to
the sensorial reaction;^† so that if one nervous portion
is excited, the sensorium is affected, which, reacting by
means of the nerves on another part, draws it into con-
sent with the first, although there exist between them no
immediate nervous connection. Such is the sympathy
of the iris, when the retina is stimulated by light; and
of the diaphragm during sneezing, when the Schnei-
derian membrane is irritated.

There are other examples of sympathy, in which the
nerves, if they have any, have a more remote and ac-
cessory, share:^‡ among these must be placed the sym-
pathy along the blood vessels, strikingly instanced
between the internal mammary and epigastric arteries,
especially in advanced pregnancy; that along the lym-
phatic vessels,^§ also most remarkable during pregnancy
and suckling; and again, that dependent on analogy of
structure and function, v.c. the sympathy of the lungs
with the surface and intestines. (A)

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57. The vital powers will be hereafter separately con-
sidered, under the distinct heads of our subject: – The
nisus formativus under the head of Generation; irrita-
bility under that of the Muscles; sensibility under that
of the Nervous System; the vita propria whenever
occasion requires.

58. Besides our former brief remarks (40) upon con-
tractility, a few more minute will at present be very
appropriate.

It prevails universally,(40) wherever the mucous tela
is discoverable.

It is consequently most abundant in parts destitute of
proper parenchyma, but composed almost entirely of
mucous tela, v.c. in certain membranes: for no one will
deny their contractility, who reflects upon the spastic
motions of the dartos, the male urethra, or the gall
bladder, which during death is always closely contracted
upon any calculi it may contain.

It appears also in those viscera which consist chiefly
of this tela, v.c. in the lungs, whose external surface
we have found on living dissection very contractile; but
by no means, as Varnier asserted, truly irritable. (B)

The presence of contractility even in the bones, is
demonstrated by the shrinking of the alveoli after the
loss of the teeth, and by the process of necrosis, by
which the new bone, when the dead portion is extricated
from its cavity, contracts to its natural size and figure.

The vitreous substance of the teeth, being destitute of
this tela (22), possesses no contractility, as I think ap-
pears from the circumstance of its not shrinking, like
the alveoli, if a portion is separated by caries or
fracture.

59. This contractility of the mucous tela is the chief
[Seite 28] cause of strength, health, and beauty; since on it depend
the vital elasticity and fulness,^* and indeed the tone of
parts, so elegantly decribed by Stahl; for by its means,
the mucous tela, to mention one only of its functions,
absorbs, during health, the serous fluid (27) like a
sponge, and propels it into the lymphatic vessels: in
disease, on the contrary, having lost its tone, it is filled
with water, giving rise to oedema and similar cachexies.

60. Finally, the great influence of this contractility
in producing the peculiar constitution and tempera-
ments of individuals, is manifest from its universal
existence, its close union with the other vital powers,
and from its infinite varieties and degrees in different
persons.

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SECT. V.
OF THE MENTAL FACULTIES.

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61. Man, whom we have found possessed of a body,
answering completely both in matter and texture, as
well as vital powers, the purposes of its formation, is
endowed likewise with a mind, a ‘“divinae particula
aurae,”’ intimately connected with the body, and deve-
loping by education and exercise various kinds of facul-
ties, which we shall concisely enumerate, as far as they
belong to our subject.^*

62. The sensibility of the nerves, mentioned above
among the vital powers, (43) constitutes, as it were, the
medium which propagates the impressions of stimuli
upon sensible parts, and especially upon the organs of
sense (to be hereafter examined), to the sensorial por-
tion of the brain, in such a manner that they are per-
ceived by the mind.

63. The mental faculty to be first enumerated, and
indeed to be placed at the bottom of the scale, is the
faculty of perception, by means of which the mind takes
cognizance of impressions made upon the body, and
chiefly upon the organs of sense, and becomes furnished
with ideas.

[Seite 31]

64. This faculty is assisted by another of an higher
order, – attention, which so directs the mind, when ex-
cited, to any idea, that it dwells upon that idea alone
and surveys it fully.

65. To preserve and recall the marks of ideas, is the
office of memory – that part of the mind, which, in the
language of Cicero, is the guardian of the rest.

66. Imagination,^* on the contrary, is that faculty of
the mind, which represents not merely the signs, but the
very images, of objects in the most lively manner, as if
they were present before the eyes.

67. Abstraction forms general notions more remote
from sense.

68. Judgment compares and examines the relations of
the ideas of sense and of abstract notions.

69. Lastly reason – the most noble and excellent of
all the faculties, draws inferences from the comparisons
of the judgment.^†

70. The combination of these constitutes the intellectual
faculty; but there is another order, relating to appetency,
the word being taken in its most extensive meaning.

[Seite 32]

71. For since we are impelled by various internal
stimuli to provide food and other necessaries, and
also to satisfy the sexual instinct, and are impelled the
more violently, in proportion as imagination inflames
our wishes, desires, properly so called, are thus pro-
duced; and if, on the other hand, the mind becomes
weary of unpleasant sensations, aversions occur.

72. Finally, that faculty which selects out of many
desires and aversions, and can at pleasure determine to
perform functions for certain purposes, is denominated
volition.

73. Our order of enumeration corresponds with that
of the development of the faculties, and with the
relation in which those termed brute – common to man
and animals, and those more or less peculiar to man,
stand to each other.

---

SECT. VI.
OF HEALTH AND HUMAN NATURE.

[Seite 35]

74. Since health,^* which is the object of physiology,
depends upon such an harmony and equilibrium of the
matter and powers of the system, as is requisite for the
due performance of its functions, it is very evident how
the four principles, examined above, contribute to its
support.

75. Fluids properly prepared are the first requisite;
in the next place, solids duly formed from the fluids;
then the invigorating influence of the vital powers; lastly,
a sound mind in this sound body.

76. These four principles act and react perpetually
upon each other: the fluids are stimuli to the solids;
these again are calculated by their vital powers to ex-
perience the influence of these stimuli, and react upon
them. In reference to the intimate union of the mind
with the body, suffice it at present to remark, that it is
far more extensive than might at first be imagined. For
instance, the influence of the will is not confined within
the narrow limits of those actions designated voluntary
in the schools of physiology; and the mind, on the
other hand, is influenced by the affections of the body, in
many other ways than by the perceptions of sense.^†

[Seite 36]

77. From the endless variety and modification of the
conditions belonging to these four principles, it may be
easily understood what great latitude^* must be given
to the notion of health. For since, as Celsus long ago
observed, every one has some part weaker than the rest,
Galen may in this sense assert with truth, that no one
enjoys perfect health. And even among those whom
we commonly regard as in good health, this is variously
modified in each individual.^†

78. Upon this endless modification is founded the
difference^‡ of temperaments; or, in other words, of the
mode and aptitude of the living solid^§ in each indi-
vidual, to be affected by stimuli, especially the mental;
and again, of the mental stimuli, to be excited with
greater or less facility.

79. So various are the differences of degree and com-
bination in the temperaments, that their divisions and
orders may be multiplied almost without end. We
[Seite 37] shall content ourselves with the four orders commonly
received.^* The sanguineous – excited most readily, but
slightly: The choleric – excited readily and violently:
The melancholic – excited slowly, but more permanently:
And the phlegmatic – excited with difficulty.

This division, although built by Galen upon an absurd
foundation borrowed from an imaginary depravation of
the elements of the blood, appears, if made to stand
alone, both natural and intelligible.

80. The predisposing and occasional causes of the
diversity of temperaments are very numerous; v.c.
hereditary tendency, habit of body, climate, diet, re-
ligion, mode of life, and luxury.^†

81. Besides the variety of temperaments, circum-
stances peculiar to every individual, by influencing the
number, as well as the energy and vigour, of the functions,
increase the latitude (77) in which the term health must
be received. In regard to age, the health of a new-born
infant is different from that of an adult; in regard to
sex, it differs in a marriageable virgin and an old woman
past child-bearing, and during menstruation and suck-
ling; in regard to mode of life, it is different in the
barbarous tribes of North America and in effeminate
Sybarites.

Moreover, in every person, custom has an extra-
ordinary influence^‡ over certain functions, v.c. sleep,
[Seite 38] diet; and has therefore acquired the name of second
nature.

82. The more functions flourish simultaneously in the
body, the more considerable is its life; and vice versâ.
Hence life is greatest when the functions have attained
their highest perfection in adult age; and least when
the functions, although very perfect, are fewer and more
sluggish, v.c. in the newly conceived embryo; life is
for the same reason less vigorous during sleep than
during the opposite state.

83. The functions have been long divided by physi-
ologists into four classes. This division, although not
unexceptionable nor exactly conformable to nature,^*
may assist the memory.^†

1. The first class comprehends the vital functions, so
termed, because their uninterrupted and complete per-
formance is necessary to life. Such are the circulation
and respiration.

2. The second comprehends the animal functions, by
which animals are chiefly distinguished from vegetables.
Such is the connection of the mind with the body, espe-
cially sense and muscular motion.

3. The third is the natural, by means of which the
body is nourished.

4. The fourth, the genital, intended for the propaga-
tion of the species.

[Seite 39]

We shall now examine each of these separately, be-
ginning with the vital.

---

SECT. VII.
ON THE MOTION OF THE BLOOD.

[Seite 60]

84. The blood, to whose great and multifarious im-
portance in the system we have slightly alluded, (16)
is conveyed, with a few exceptions, (5) into the most
internal and extreme recesses. This is proved by the
minute injection of the vessels, and by the well known
fact of blood issuing from almost every part on the
slightest scratch.

85. This purple fluid does not, like an Euripus, ebb
and flow in the same parts, as the ancients imagined,
but pursues a circular course; so that being propelled
from the heart into the arteries, it is distributed through-
out the body, and returns again to the heart through
the veins.^*

86. We shall, therefore, say something at present
of the vessels which contain the blood; and afterwards,
of the powers by which they propel and receive it.

87. The vessels which receive the blood from the
heart and distribute it throughout the body, are termed
arteries. These are upon the whole less capacious than
[Seite 61] the veins; but in adult and advanced age especially,
of a texture far more solid and compact, very elastic
and strong.

88. The arteries consist of three coats:^*

I. The exterior, called, by Haller, the tunica cellu-
losa propria; by others, the nervous, cartilaginous, ten-
dinous, &c. It is composed of condensed cellular
membrane, externally more lax, internally more and
more compact: blood vessels are seen creeping upon
it:^† it gives tone and elasticity to the arteries.

II. The middle coat consists of transverse fibres,^‡
lunated or falciform, and almost of a fleshy nature:
hence this has the name of muscular coat, and ap-
pears to be the chief seat of the vital powers of the
arteries.

III. The inner coat lining the cavity of the arteries
is highly polished and smooth. This is much more dis-
tinct in the trunks and larger branches than in the
smaller vessels.

89. Every artery originates either from the pulmo-
nary artery (the vena arteriosa of the ancients), which
proceeds from the anterior ventricle of the heart and
goes to the lungs; or from the aorta, which proceeds
from the posterior ventricle and is distributed through-
out the rest of the system. These trunks divide into
branches, and these again into twigs.

90. According to the commonly received opinion,
[Seite 62] the united capacity of the branches is greater than that
of the trunk from which they arise. But I fear that
this is too general an assertion, and even that the mea-
sure of the diameter has been sometimes improperly
confounded with that of the area. I myself have never
been able to verify it, although my experiments have
been frequently repeated, and made, not on vessels in-
jected with wax, but on the undisturbed vessels of
recent subjects, on the innominata and its two branches
– the right carotid and subclavian, on the brachial and
its two branches – the radial and ulnar.^*

The inconstancy of the proportion between the capa-
city of the branches and trunks is clearly shewn by the
various size of the vessels under different circum-
stances, v.c. by the relative capacity of the inferior
thyroid artery in the infant and the adult; of the epi-
gastric artery and also of the uterine vessels in a virgin
and a woman far advanced in pregnancy; of the
omental vessels during the repletion and vacuity of the
stomach.^†

91. The arteries, after innumerable divisions and
important anastomoses^‡ connecting different branches,
terminate at length in the beginning of the veins. By
this means, the blood is conveyed back again to the
heart. The distinction between artery and vein at the
point of union, is lost.

[Seite 63]

In the present state of our knowledge, the umbilical
vessels are to be regarded as the only exception to the
termination of arteries in veins. We shall shew that
they are connected with the uterine vessels by the in-
tervention of a spongy substance, called parenchyma.

92. Another description of vessels arise universally
from the arteries and are called colourless, from not con-
taining pure blood, either on account of their minute-
ness, or of their specific irritability which causes them
to reject that fluid. These are the nutrient and other
secretory vessels: of which hereafter.

93. The blood conveyed from the heart by the arte-
ries is carried back by the veins.

These are very different in function and structure
from the arteries, excepting however the minutest of
both systems, which are indistinguishable.

94. The veins, excepting the pulmonary, are upon
the whole more capacious than the arteries; are more
ramified; much more irregular in their course and divi-
sion; in adult age, softer and more elastic, but still
very firm and remarkably expansile.

95. Their coats are so much thinner that the blood
appears through them. They are likewise less in num-
ber, being solely a cellular external, somewhat resem-
bling the nervous of the arteries; and a very polished
internal, also nearly agreeing with that of the arteries.

A muscular coat exists in the largest trunks only.

96. The interior coat forms, in most veins of more
than a line in diameter, very beautiful valves, of easy
play, resembling bags, generally single, frequently dou-
ble, and sometimes treble, so placed, that the fundus
lies towards the origin of the vein, the limbus towards
the heart.

[Seite 64]

These valves are not found in some parts; not in the
brain, heart, lungs, secundines, nor in the system of
the vena portae.

97. The twigs, or, more properly, the radicles, of the
veins, unite into branches, and these again into six
principal trunks: viz. into the two cavae, superior and
inferior; and the four trunks of the pulmonary vein (the
arteria venosa of the ancients).

The vena portae is peculiar in this, that, having en-
tered into the liver, it ramifies like an artery, and its
extreme twigs pass into the radicles of the inferior
cava, thus coalescing into a trunk.

98. That the blood may be properly distributed and
circulated through the arteries and veins, nature has
provided the heart,^* in which the main trunks of all
the blood vessels unite, and which is the grand agent
and mover of the whole system, – supporting the chief
of the vital functions with a constant and truly wonder-
ful power, from the second or third week after concep-
tion to the last moment of existence.

99. The heart alternately receives and propels the
blood. Receiving it from the body by means of the
superior and inferior vena cava, and from its own sub-
stance through the common valvular^† orifice of the
coronary veins, it conveys that fluid into the anterior
sinus and auricle; and thence into the corresponding
ventricle, which, as well as the auricle, communicates
[Seite 65] with both orders of its own vessels by the openings of
Thebesius.^*

100. From this anterior, or, in reference to the heart
of some animals, right, ventricle, the blood is impelled
through the pulmonary artery into the lungs: returning
from which, it enters the four pulmonary veins and pro-
ceeds into their common sinus and the left, or, as it is
now more properly termed, posterior auricle.^†

101. It flows next into the corresponding ventricle;
and then passing into the aorta, is distributed through
the general arterial system and the coronary vessels of
the heart.^‡

102. Having proceeded from the extreme twigs of
the general arterial system into the radicles of the
veins, and from the coronary arteries into the coronary
veins, it finally is poured into the two venae cavae, and
then again pursues the same circular course.

103. The regularity of this circular and successive
motion through the cavities of the heart is secured,
and any retrograde motion prevented, by the valves
which are placed at the principal openings, viz. at the
openings of the auricles into the ventricles, and of the
ventricles into the pulmonary artery and aorta.

104. Thus the ring, or venous tendon, which forms the
limit of the anterior auricle and ventricle, descending
into the latter cavity, becomes these tendinous valves.^§
These were formerly said to have three apices, and
[Seite 66] were therefore called triglochine or tricuspidal: they
adhere to the fleshy pillars, or, in common language,
the papillary muscles.

105. In a similar manner, the limits of the posterior
auricle and ventricle are defined by a ring of the same
kind, constituting two valves, which, from their form,
have obtained the appellation of mitral.^*

106. At the opening of the pulmonary artery^† and
aorta^‡ are found the triple semilunar or sigmoid valves,^§
fleshy and elegant, but of less circumference than the
mitral.

107. It is obvious how these valves must prevent
the retrocession of the blood into the cavae. They
readily permit the blood to pass on, but are expanded,
like a sail, against it, by any attempt at retrograde
movement.

108. The texture of the heart is peculiar: fleshy, in-
deed, but very dense and compact, far different from
common muscularity.^ǁ It is composed of fasciculi of
fibres, more or less oblique, here and there singularly
branching out, curiously contorted and vorticose in
their direction, lying upon each other in strata, closely
interwoven between the cavities, and bound by four
cartilaginous bands to the basis of the ventricles, which
are thus supported and distinguished in their texture
from the fibres of the auricles.^**

[Seite 67]

109. These fleshy fibres are supplied with very soft
nerves^* and an immense number of blood vessels,
which arise from the coronary arteries, and are so in-
finitely ramified,^† that Ruysch described the whole
structure of the heart as composed of them.^‡

110. The heart is loosely contained in the pericar-
dium.^§ This is a membraneous sac, arising from the
mediastinum, very firm, of the same figure as the heart,
and moistened by an exhalation from the arteries of that
organ. Its importance is evinced by its existence being,
in red blooded animals, as general as that of the heart;
and by our having only two instances on record of its
absence in the human subject.^ǁ

111. By this structure, the heart is adapted for per-
petual and equable motions, which are an alternate
systole and disastole, or contraction and relaxation of
the auricles and ventricles in succession.

112. Thus, as often as the auricles contract to impel
the blood of the venae cavae and pulmonary veins into
the ventricles, these are at the same moment relaxed,
to receive the blood: immediately afterwards, when
the distended ventricles are contracting to impel the
blood into the two great arteries, the auricles relax
and receive the fresh venous supply.

[Seite 68]

113. The systole of the ventricles, upon which is
said to be spent one third of the time of the whole ac-
tion of the heart, is performed in such a way, that their
external portion is drawn towards their septum, and
the apex of the heart towards the base.^* This at first
sight seems disproved by the circumstance of the apex
striking against the left nipple and consequently ap-
pearing elongated, – a circumstance, however, to be
attributed to the double impetus of the blood flowing
into the auricles and expelled from the ventricles, by
which the heart must be driven against that part of the
ribs. (A)

114. The impulse imparted by the heart to the blood,
is communicated to the arteries, so that every systole
of the heart is very clearly manifested in those arteries
which can be explored by the fingers and exceed 1/6 of
an inch in diameter, and in those also whose pulsation
can be otherwise discovered, as in the eye and ear.
The effect upon the arteries is called their disastole,
and is correspondent and synchronous with the systole
of the heart.

115. The quickness of the heart’s pulsations during
health varies indefinitely; chiefly from age, but also
from other conditions which at all ages form the pecu-
liar health of an individual; so that we can lay down
no rule on this point. I may, however, be permitted
to mention the varieties which I have found in our
climate^† at different ages, beginning with the new-born
[Seite 69] infant, in which, while placidly sleeping, it is about
140 in a minute.

In those more advanced, I have scarcely twice found
it alike.

116. The pulse is, caeteris paribus, more frequent
in women than men, and in short than tall persons.
A more constant fact, however, is its greater slowness
in cold climates.^* Its greater frequency after meals
and coition, during continued watchfulness, exercise,
or mental excitement, is universally known. (B)

117. The heart rather than the arteries is to be re-
garded as the source of these varieties.

Its action continues in this manner till death, and
then all its parts do not, at once, cease to act; but the
right portion, for a short period, survives the left.^†

For since the collapsed state of the lungs impedes
the course of the blood from the right side, and the
veins must be turgid with the blood just driven into
[Seite 70] them from the arteries, it cannot but happen that this
blood, driving against the right auricle, must excite it
to resistance for some time after the death of the left
portion of the heart.

118. This congestion on the right side of the heart
affords an explanation of the small quantity of blood
found in the large branches of the aorta. Weiss,^* and
after him Sabatier,^† ascribe to this cause likewise the
comparatively larger size^‡ of the right auricle and
ventricle in the adult dead subject especially.

119. The motion of the blood is performed by these
two orders of vessels in conjunction with the heart. Its
celerity in health cannot be determined: for it varies
not only in different persons, but in different parts of
the same person. Generally, the blood moves more
slowly in the veins than in the arteries, and in the small
vessels than in the large trunks. But these differences
have been overrated by physiologists.

The mean velocity of the blood flowing into the
aorta, is usually estimated at eight inches for each pul-
sation, or at fifty feet in a minute.

120. Some have affirmed that the globules of the
cruor move more in the axis of the vessels, and with
greater rapidity, than the other constituents of the
blood. I know not whether this rests upon any
satisfactory experiment, or upon an improper appli-
cation of the laws of hydraulics; improper, because
[Seite 71] it is absurd to refer the motion of the blood through
living canals, to the mere mechanical laws of water
moving in an hydraulic machine. I have never ob-
served this peculiarity of the globules. My persuasion
is still more certain that the globules pass on with the
other constituents of the blood, and are not rotated
around their own axis, – that besides the progressive,
there is no intestine, motion in the blood; although in-
deed there can be no doubt that the elements of this
fluid are occasionally divided, where it is variously
impelled according to the different direction, division,
and anastomoses of the vessels.

121. The powers of the sanguiferous system are now
to be examined: first, those of the heart, by far the
greatest of all; afterwards, those which are only sub-
sidiary, though indeed highly useful.

122. That the powers of the heart cannot be accu-
rately calculated is clear, upon reflecting that neither
the volume of the blood projected at each pulsation,
nor the celerity nor distance of its projection, much
less the obstacles to the powers of the heart, can be
accurately determined.

123. A rough calculation may be made by taking
every probable conjecture together: v.c. if the mean
bulk of the blood is considered as 10 pounds, or 120
ounces; the pulsations 75 in a minute, or 4500 in an
hour; and the quantity of blood expelled from the left
ventricle at each contraction, as two ounces; it fol-
lows that all the blood must pass through the heart
75 times every hour. The impetus of the blood pass-
ing from the heart, may be conceived by the vio-
lence and altitude of the stream projected from a
large wounded artery situated near it. I have seen
[Seite 72] the blood driven to the distance of at least live feet
from the carotid of an adult and robust man.^*

124. This wonderful, and, while life remains, con-
stant, strength of the heart, is universally allowed to
depend on its irritability, (41) in which it very far sur-
passes, especially as to duration,^† (98) every other
muscular part.^‡

That the parietes of the cavities are excited to con-
traction by the stimulus of the blood, is proved by the
experiment of Haller, who lengthened at pleasure the
motion of either side of the heart, by affording it
the stimulus of the blood for a longer period than the
other.^§ (C)

[Seite 73]

125. Since a supply of nerves and blood is requisite
to the action of the voluntary muscles, it has been
enquired whether these are requisite to the heart also.^*

The great influence of the nerves over the heart, is
demonstrated by the size of the cardiac nerves, and
by the great sympathy between the heart and most func-
tions, however different. A convincing proof of this,
is the momentary sympathy of the heart during the
most perfect health^† with the passions, and with the
primae viae in various disorders. But the great import-
ance of the blood to the irritability of the heart, is evi-
dent from the great abundance of vessels in its mus-
cular substance.

Nevertheless it is very probable, that the importance
of the nerves in this respect is greater in the voluntary
muscles, and of the blood in the heart.

126. Besides these powers of the heart, there is ano-
ther, which is mechanical, dependent on structure, and
greatly contributing, in all probability, to sustain the
circulation. For when the blood is expelled from the
contracted cavities, a vacuum takes place, into which,
according to the common laws of derivation, the neigh-
bouring blood must rush, being prevented, by means
of the valves, from regurgitating.^‡ (D)

[Seite 74]

127. We must now enquire what powers are exerted
by other organs in assisting the circulation. The exist-
ence of some secondary powers and their ability to as-
sist, or even in some cases to compensate for, the
action of the heart, are proved by several arguments:
v.c. the blood moves in some parts to which the in-
fluence of the heart cannot reach, – in the vena portae
and placenta; not to mention instances of the absence
of the heart.^*

128. The principal of these powers is the function of
the arteries, not easy indeed to be clearly understood
and demonstrated. 1. They have a muscular coat. (E)
2. That they are irritable, has been proved by repeated
experiments.^† 3. The size of the soft nerves arising
from the sympathetic, and surrounding the larger arte-
rial branches, particularly in the lower part of the
abdomen,^‡ argues the importance of these vessels in
assisting the motion of the blood.^§

129. The arteries pulsate, and indeed violently, so
[Seite 75] that if, v.c. we place one leg over the other knee, we
find not only that it, but even a much greater weight,
may be raised by the pulsation of the popliteal. Hence
an alternate systole and diastole, corresponding with
those of the heart, have long been assigned to them.
But this, although commonly believed on the evidence
of sense, is open to much question:^* it may be asked,
especially, whether this pulsation is referrible to the
power of the artery, or only to the impulse given by
the heart to the blood propelled into the aorta.

130. And indeed, after all, it appears that the dias-
tole of an artery is owing to the blood, – to a lateral
distention given by the impetus of the blood, so that
the coats are expanded; and the vessel, by its elasti-
city, the next moment reacquires its natural thickness.
To the same impulse may be ascribed the lateral mo-
tion of the axis, observable in the larger arteries, if
serpentine and lying in loose cellular substance. (F)

The genuine systole, produced by a contraction of
their substance, scarcely occurs, probably, while the
heart acts with sufficient vigor; but when they are un-
usually stimulated, or if the action of the heart fails or
is impeded by severe disease, then indeed the arteries
may supply its place and propel the blood by their own
vital energy,

131. Since Whytt^† and other illustrious physiolo-
[Seite 76] gists have been convinced that the influence of the
heart could not reach the extreme arteries and the ori-
gins of the veins, they have ascribed the progression
of the blood in those vessels to a kind of oscillation,
and have happily employed this to demonstrate the
nature of inflammation. Many kinds of phenomena,
both physiological, as those regarding animal heat,
and pathological, as those observed in spasms and
particularly in fevers, favour the supposition of this
oscillatory faculty, though it is not demonstrable to
the eye. (G)

132. It remains now to enquire into the aid given to
the returning blood by the veins, not alluding at all to
their radicles. We should conclude at first sight that
they have less active power^* than the rest of the san-
guiferous system, and that the return of their purple
blood to the heart is chiefly ascribable to the impetus
a tergo of the arterial blood, and to their valvular
structure which prevents any reflux. The efficacy
of the valves in this point of view, is shewn by the
distentions and infarctions of the veins in the lower
part of the abdomen, which are found destitute of
valves.^†

The existence of vital powers in the venous trunks
is probable,^‡ from the example of the liver and pla-
[Seite 77] centa (127), and from experiments instituted on living
animals. We formerly mentioned the muscular ap-
pearance in the extreme veins near the heart (95). (H)

133. These are the chief powers which move the
blood and depend upon the structure and vitality of the
sanguiferous system: we say nothing of the effect of
gravity, attraction, and other powers, common to all
matter. The more remote assistance derived after
birth from particular functions, v.c. respiration and
muscular motion, will appear in our account of those
functions.

---

SECT. VIII.
OF RESPIRATION AND ITS PRINCIPAL USE.

[Seite 84]

134. The lungs,^* closely connected with the heart
both by proximity and by relation of function, are two
viscera, large after birth, so light as to swim in water,
and composed of a spongy, and, as it were, spumous,
but pretty tenacious, parenchyma.^†

135. They fill each cavity of the chest, and are con-
tiguous to the sacs of the pleurae, to which, as well as
to the other contents of the thorax, they model and
apply themselves. (A)

136. They, in a manner, hang from the wind-pipe
usually called the aspera arteria, which, besides its
interior coat always smeared with mucus, and the sub-
jacent very sensible nervous coat, consists of another
which is muscular, surrounding the latter, and divided,
except posteriorly, by an indefinite number of cartila-
ginous falciform arches.

137. The aspera arteria, having entered the thorax,
is bifurcated into the two bronchiae, and these, the
more deeply they penetrate into the lobes and lobules
of the lungs, are the more and more ramified, losing
[Seite 85] both their cartilaginous rings and muscular coat, until
their extreme divisions terminate in those cells which
form the chief part of the substance of the lungs and
alternately receive and emit the air we breathe.

138. The shape and magnitude^* of the air cells are
various. The former is generally polyedrical. The
latter, in regard to surface, is scarcely to be defined:^†
though, indeed, the capacity of the lungs of an adult,
during a strong inspiration, is about 120 cubic inches.
The immense size to which the lungs may be inflated,
when the chest has been opened, has no relation to our
present subject.

139. The cells are invested and connected by the
common but delicate mucous web, – the general vincu-
lum of the body, and must be carefully distinguished
from it. In healthy and very recent lungs, I have found
the cells so unconnected that they were distended in
one insulated spot by air cautiously inflated into a fine
branch of the bronchiae, while neither the neighbouring
cells nor the cellular membrane, which lies between
the cells, admitted a single portion. If air is forcibly
thrown in, the air cells are ruptured and confounded
with the cellular membrane, and both parts distended.

140. The mucous web surrounding the air cells of
the lungs is supplied with innumerable blood vessels –
divisions of the pulmonary artery and four pulmonary
veins, the branches of which accompany the ramifica-
tions of the bronchiae,^‡ and, after repeated division,
[Seite 86] form at length a most delicate and immense collection
of reticulated anastomoses. This extraordinary net-
work, penetrating the mucous web on every side,
closely surrounds the air cells, so that the prodigious
quantity of blood existing in the pulmonary vessels is
separated from the contact of the air by very fine mem-
branes only which Hales estimated as scarely 1/1000 of
an inch in thickness.

141. As each ramification of the bronchiae possesses
a peculiar bunch or lobule of air cells, (139) so again
each of these possesses a peculiar system of blood
vessels, the twigs of which anastomose in the net-work
with one another, but scarcely at all with the blood-
vessels of the other lobules, as is proved by micros-
copic observations on living frogs and serpents, by
minute injections, and by the phenomena of vomicae
and other local diseases of the lungs.

142. The common membrane investing the lungs
is the chief seat of a remarkable net-work of lymphatic
vessels^* which run to numerous lymphatic or conglo-
bate glands,^† carefully to be distinguished from a
neighbouring order of glands, called bronchial, that
are supplied with an excretory duct and are of the
conglomerate kind.^‡

143. The thorax, which contains the lungs, has an
osseous and cartilaginous base, somewhat resembling
a bee-hive, throughout very firm and stable, but in
every part more or less moveable for the purpose of
respiration.^§

[Seite 87]

This holds good chiefly with the six pairs of true ribs
below the first pair, each of which is more moveable
than the one above in proportion to the greater length
both of its own body and of its cartilaginous appendix.
The cartilages are united by a kind of amphiarthrosis
to the margin of the sternum on each side. (B)

144. Between the edges of the ribs lie two strata of
intercostal muscles, differing in the direction of their
fibres, but conspiring to produce the same motion.

At the base of the thorax, the diaphragm^* is sub-
tended in the form of an arch. It is a considerable
muscle, and, in the words of Haller, next in importance
to the heart. Its utility in the mechanical part of res-
piration was long since shewn, by the excellent expe-
riments of Galen^† upon living animals, to depend
chiefly on the phrenic nerve.^‡

Its antagonists are the abdominal muscles, especially
the two oblique and the transverse.

145. The thorax thus constituted, is, after birth,
dilated by inspiration and subsequently reduced to a
smaller capacity by expiration.

During the former act, the thorax is enlarged late-
[Seite 88] rally and inferiorly, so that the bodies of the six ribs
mentioned above (143) are elevated and their inferior
margin drawn somewhat outwards, and the arch of
the diaphragm at the same time rather depressed and
flattened.

I have never observed the inferior extremity of the
sternum, in tranquil respiration, to be thrust forwards,
as some have asserted. (C)

146. This alternate motion of the chest continues,
during health and freedom from restraint, from the
hour of birth till death. Its object is, that the lungs
may be expanded to admit the air and contracted to
expel it, in perpetual alternation. This alternation
occurs, in an adult at rest, about 14 times in a minute,
– once to about five pulsations of the heart.

147. For man, in common with all warm-blooded
animals, cannot long retain the inspired air, but is
compelled to discharge it and take in a fresh supply
of this pabulum of life, as it always has been denomi-
nated.^* Common observation teaches, that however
pure may be the air entering the lungs, it instantly
undergoes remarkable changes, by which it is con-
taminated and rendered unfit for another inspiration,
unless it is renewed.^†

[Seite 89]

148. It may be asked what are the changes which
the air experiences during inspiration, and which con-
sist not in the loss of elasticity, as was formerly
imagined, but in the decomposition of its elements.^*
For the atmospheric air which we breathe, is a singular
mixture of constituents, differing very much in their
nature from each other; and, not to mention hetero-
geneous matters, such as odorous effluvia, various
exhalations, and innumerable others which are gene-
rally present, is always impregnated with aqueous
vapour, electric and magnetic matter, and generally
with carbonic acid gas; and is itself composed of
unequal parts of two aëriform fluids, viz. 79 of azotic
gas, and 21 of oxygen gas in 100.

149. In the first place we know for certain, that at
every inspiration (the fulness of which varies infinitely
in different men of the same age, breathing placidly^†),
besides the quantity of azotic gas being somewhat
diminished,^‡ the oxygen gas is in a great measure
converted into carbonic acid gas, or fixed air; so that
the air of expiration, if collected, instantly extinguishes
flame and live coals, precipitates lime from lime water,
and is specifically heavier than atmospheric air, and
rendered unfit for respiration;^§ it also contains much
[Seite 90] aqueous vapour, which is condensed in a visible form
by a temperature of 60° of Fahr.^*

150. It is therefore probable, that, during inspira-
tion, the base of the oxygenous portion is set at liberty,
and, being united with the arterial blood, is conveyed
throughout the system; while the carbon and hydrogen
are brought back with the venous blood to the right
side of the heart, and thrown off like smoke, as the
ancients expressed it, in the lungs.^†

The more florid colour of the arterial blood,^‡ the
darker of the venous, and the analogous appearance of
the blood, if exposed to the gases in question, (13)
correspond admirably with this theory. Some diffi-
culties, indeed, remain to be solved, v.c. how the
carbon can be united in the lungs with the oxygen, so
as to fly off in the form of carbonic acid gas. (C)^§

151. This perpetual change of elements occurring in
[Seite 91] respiration after birth, we shall show to be very diffe-
rently accomplished in the foetus, viz. by means of the
connection of the gravid uterus with the placenta.
But when the child is born and capable of volition, the
congestion of blood that takes place in the aorta, from
the obstruction in the umbilical arteries; the danger of
suffocation from the cessation of those changes of the
blood, in regard to oxygen and carbon, (13) hitherto
produced in the uterine placenta; the novel impression
of that element into which the child, hitherto an aquatic
being, is conveyed; the cooler temperature to which it
is now exposed; and the many new stimuli which are
now applied, seem to induce new motions in the body,
especially the dilatation of the chest and the first
inspiration.

The lungs being for the first time dilated by inspi-
ration, open a new channel to the blood, so that,
being obstructed in the umbilical arteries, it is derived
to the chest.

Since the inspired air becomes hurtful and unplea-
sant to the lungs by the decomposition which it expe-
riences, I should ascribe to the most simple corrective
powers of nature, the subsequent motion by which the
poisonous mephitis, as it may be called, is expelled
and exchanged for a fresh supply.

The consideration of all these circumstances, espe-
cially if the importance of respiration to circulation,
demonstrated by the well-known experiment of Hooke,^*
[Seite 92] be remembered, will, in my opinion, explain the cele-
brated problem of Harvey^* better^† than most other
attempts of physiologists. (D)^‡

---

SECT. IX.
OF THE VOICE AND SPEECH.

[Seite 99]

152. We have described the chief use of respiration.
We shall hereafter mention how far it contributes to the
conversion of the chyle into blood, and to the support
of almost the whole class of natural functions. Its
other uses are at present to be considered.

And first, respecting the voice.^* This takes place
after birth, and proceeds from the lungs, as was ob-
served long ago by Aristotle, who called those animals
only vocal, which breathed by means of lungs. The
voice is, properly speaking, a sound, formed, by means
of expiration, in the larynx, which is a most beauti-
fully constructed organ, fixed upon the top of the
windpipe, like a capital upon a pillar.^†

153. The larynx is composed of various cartilages,
which being united together in the form, as it were,
of a little box,^‡ and supplied with a considerable and
wonderful apparatus of muscles,^§ may be moved alto-
gether, or separately, according to the variations of
the voice.

[Seite 100]

154. The part of the larynx most concerned in pro-
ducing the voice, is the glottis, or narrow opening of
the windpipe, having the epiglottis suspended, and, in
a manner, fixed upon it. It is clearly ascertained, that
the air, expired from the lungs, and striking properly
upon the margins of the glottis, becomes sonorous.

155. But it has been disputed what changes the
glottis undergoes in modulating the voice: whether it
is alternately widened and constricted, as Galen and
Dodart supposed, or whether, according to Ferrein,
the variations of voice are effected rather by the tension
and relaxation of its ligaments.

The latter, consistently with his opinion, compared
the larynx to a violin; the former, more consistently
with nature, to a flute.^*

Every thing considered, we must conclude that the
glottis, when sounding, experiences both kinds of
changes; since the grave and acute modulation of the
voice must depend very much upon the alterations
produced in the glottis by the ligaments, especially the
inferior thyreo-arytenoids – the vocal chords of Ferrein,
and by the corresponding modification of the sinuses or
ventricles of the larynx.^†

156. That every degree of motion in the glottis is
directed by the numerous muscles of the larynx, is
[Seite 101] proved by the beautiful experiment of tying or dividing
the recurrent nerves, or par vagum,^* and thus weaken-
ing or destroying the voice of the animal. (A)

157. Man and singing birds have the power of
whistling. In the latter, it is accomplished by a larynx
placed at each extremity of the wind-pipe and divided
into two portions. The former, though possessing a
single and undivided larynx, has only learned, I ima-
gine, to imitate birds by the coarctation of his lips.^†

158. Singing, which is compounded of speech and an
harmonic modulation of the voice, I conceive to be
peculiar to man and the chief prerogative of his vocal
organs. The power of whistling is innate in birds;
many of them may easily be taught to pronounce
words, and instances have been known of this even in
dogs. But it is recorded, that genuine singing has
once or twice only, and then indeed but indifferently
and with the utmost difficulty, being taught to parrots;
while, on the other hand, scarcely a barbarous nation
exists, in which singing is not common.^‡

[Seite 102]

159. Speech is a peculiar modification of the voice,
adjusted to the formation of the sounds of letters by
the expiration of air through the mouth or nostrils, and
in a great measure by the assistance of the tongue,
applied and struck against the neighbouring parts, the
palate and teeth in particular, and by the diversified
action of the lips.^* (B)

The difference between voice and speech is there-
fore evident. The former is produced in the larynx;
the latter by the singular mechanism of the organs
above described.

Voice is common to both brutes and man, even
immediately after birth, nor is entirely absent in those
wretched infants who are born deaf. But speech fol-
lows only the culture and employment of reason, and
is consequently, like it, the privilege of man in dis-
tinction to the rest of animal nature. For brutes,
natural instinct is sufficient: but man, destitute of this
and other means of supporting his existence inde-
pendently, enjoys the prerogative of reason and lan-
guage; and following, by their means, his social des-
tination, is enabled to form, as it were, and manifest
his ideas, and to communicate his wants to others, by
the organs of speech.

160. The mechanism^† of speech and articulation is
[Seite 103] so intricate and so little understood, that even the
division of letters and their distribution into classes^*
is attended with much difficulty.

The division, however, of Ammann,^† into vowels,
semi-vowels, and consonants, is very natural:

I. He divides the vowels^‡ into simple – a, e, i, y, o, u,
and mixed – ä, ö, ü.

These are formed by merely the voice.

The semi-vowels and consonants are articulated by
the mechanism of speech.

II. The semi-vowels are nasal – m, n, ng (n before g,
which is nearly related to it), that is, the labio-nasal m,
the dente-nasal n, and the gutture-nasal ng; or oral
(lingual) – r, l, that is, r with a vibration of the tongue,
or l with the tongue less moved.

III. The consonants are distinguished into hissing
(pronounced in succession)-h, g, ch, s, sch, f, v, ph,
that is h, – formed in the throat, as it were a mere aspi-
ration; g and ch – true consonants; s, sch, – produced
between the teeth; f, v, ph, – formed by the applica-
tion of the lower lip to the upper front teeth: and
explosive (which are, in a manner, at once exploded,
by an expiration, for some time suppressed or inter-
[Seite 104] rupted) that is, k, q, – formed in the throat; d, t, – about
the teeth; p, b, – near the lips; and double (com-
pound) – x, z.

161. We must just mention certain other modifica-
tions of the human voice, of which some, as hiccup
and cough, belong more properly to pathology than to
physiology, but are very common in the most healthy
persons; and others, as weeping and laughing, appear
peculiar to the human race.

162. Many of these are so closely allied, as fre-
quently to be converted into each other; most also are
variously modified.

In laughter there is a succession of short and abrupt
expirations.^*

Coughing is a quick, violent, and sonorous expira-
tion, following a deep inspiration.^†

Sneezing, generally the consequence of an irritation
of the mucous membrane of the nostrils, is a violent
and almost convulsive expiration, preceded by a short
and violent inspiration.^‡

Hiccup, on the contrary, is a sonorous, very short,
and almost convulsive, inspiration, excited by an un-
usual irritation of the cardia.^§

In weeping there are deep inspirations, quickly alter-
nating with long and occasionally interrupted expi-
rations.^ǁ

[Seite 105]

Sighing is a long and deep inspiration, and the
subsequent expiration is sometimes accompanied by
groaning.^*

Nearest in relation to sighing is gaping,^† which is
produced by a full, slow, and long, inspiration, followed
by a similar expiration, the jaws at the same time
being drawn asunder, so that the air rushes into the
open fauces and the Eustachian tubes. It occurs from
the blood passing through the lungs too slowly; v.c.
when the pressure of the air is diminished, as upon
very high mountains. A peculiar feature of gaping is
the propensity it excites in others to gape likewise;
arising, no doubt, from the recollection of the pleasure
it produced. (C)

---

SECT. X.
OF ANIMAL HEAT.

[Seite 109]

163. Man, the mammalia, and birds, are distin-
guished by the natural temperature^* of their bodies
greatly exceeding that of the medium in which they are
accustomed to exist. Man is again distinguished from
these classes of animals by possessing a much lower
temperature than they; so that in this climate it is
about 96° of Fahr. while in them, and especially in
birds, it is considerably higher.^†

164. This natural temperature in man, is so con-
stant, equable,^‡ and perpetual, that, excepting slight
differences from variety of constitution, it varies but
little even in the coldest climate and under the torrid
zone. For the opinion of Boerhaave, – that man can-
not live in a temperature exceeding his own, has been
refuted, since the admirable observations^§ of H. Ellis,
[Seite 110] the celebrated traveller and formerly the captain of the
George, by the remarkable experiments^* of many
excellent physiologists.^† This striking prerogative of
man is evinced by his being restricted to no climate,
but inhabiting every part of the earth from Hudson’s
bay, where Mercury freezes, and from Nova Zembla,
to the scorching shore of Senegal.

165. The explanation of this circumstance is equally
simple and natural, and founded on the doctrine which
makes the lungs the grand receivers or focus, and
the decomposition of the oxygenised portion of the
air (148) the source or fomites, of our heat.

166. For, as the oxygenous part of the inspired air
is decomposed in the air-cells of the lungs, in such a
way that its base, which by its union with latent caloric
was before aëriform, now separates from this caloric,
it would appear that, by this decomposition, one por-
tion of the caloric is rendered sensible in the bronchiae,
while the other enters in a latent form into the blood,
circulating in the innumerable and delicate net-works
of the pulmonary vessels.^‡

167. When the oxygenised blood thus charged with
latent heat circulates through the aortic system, it
acquires carbon in the small vessels and sets free much
[Seite 111] of the latent heat which it had received: in this way is
our animal heat principally produced.^*

168. Its production and regulation, however, appear
much influenced by the secretion of fluids from the
blood, both those which are liquid and destined to
solidify by assimilation and nutrition, and those which
are permanently elastic.

169. Since those changes are effected by the energy
of the vital powers, the great influence of these upon
our temperature must be easily perceived.^†

170. Many arguments render it probable, that the
action of the minute vessels, and the conversion of
oxygenised into carbonised blood, are dependent
upon the varied excitement or depression of the vital
principle.

For the remarkable phenomena of the stability of
our temperature,^‡ (proved by the thermometer, and not
by the sense of touch, which may be fallacious) – that
[Seite 112] it is scarcely increased by the heat of summer, or dimi-
nished by the cold of winter, but found sometimes even
to increase on immersion in cold water,^* demonstrate
that the action of the minute vessels varies according
to the temperature of the medium in which we are
placed: so that, when exposed to a low temperature
(by which their tone is probably augmented) more oxy-
gen is exchanged for carbon and more heat evolved,
while in a high and debilitating temperature this ex-
change is diminished and less heat evolved.^†

171. The corium, which covers the body, and the
internal surface of the alimentary canal, eminently con-
tribute, if we are not much mistaken, to regulate our
temperature. For both these organs are supplied with
an immense number of blood-vessels, being analogous in
this respect to the lungs, and are so intimately con-
nected with the lungs by means of sympathy,^‡ as in
some degree to perform a part, and occasionally the
whole, of some of their functions in their room. This
is exemplified in adults labouring under nearly total
consumption, or other violent affections, of the lungs,
and nevertheless, existing for a length of time almost
without respiration.^§

[Seite 113]

172. This opinion respecting the action of the cuta-
neous vessels in exciting, moderating, or almost extin-
guishing, our heat, receives much support from the
physiological and pathological facts of some parts being
frequently of a higher or lower temperature than the
rest of the system.

Thus we must attribute the coldness of the dog’s
nose to the specific action of its own vessels being
modified differently from that of the rest; so on the
other hand, the burning sometimes of the cheeks and
sometimes of the palms of the hands in hectic fever, to
the locally increased action of the vessels; besides
other phenomena of the same description, v.c. the
heat of the genitals during the venereal oestrum, and
the obstinate coldness of the feet in certain diseases.

173. The alimentary canal is the only internal part,
besides the lungs, exposed to the contact of the atmos-
phere. There is scarcely occasion to prove that it is
so exposed, and that we swallow a considerable quan-
tity of air.

The air, when swallowed, is decomposed in the sto-
mach and intestines, so that, during health, it soon
loses its elastic form: not, however, when the capil-
laries of the canal are debilitated, nor when it exists
in too great quantity.

The immense congeries of blood-vessels in the intes-
tines on their internal surface which is usually thought
equal to the external surface of the body, agrees very
well with this idea.

---

SECT. XI.
OF PERSPIRATION.

[Seite 118]

174. The functions of the skin, which affords a cover-
ing to the body, are so extremely various that they
cannot all be easily described with advantage in one
chapter, but each will be considered far more conve-
niently under that class of actions to which it belongs.

For, in the first place, the skin is the organ of touch,
and will be examined in this view, under the head of
animal functions.

It is an organ of inhalation, and in this point of view
belongs to the absorbent system, to be spoken of among
the natural functions.

It is likewise the organ of perspiration, and on this
account related in many ways to the function of respi-
ration, and may, we think, very properly follow it in
this place,

175. The skin consists of three membranes – The co-
rium, internal; the cuticle, external; and the reticulum,
intermediate.

176. The cuticle or epidermis^* forms the external
covering of the body, is separable into lamellae,^† and
exposed to the atmosphere, the contact of which can
be borne by scarcely any other part, if you except the
[Seite 119] enamel of the teeth. For this reason, the internal cavi-
ties and the canals which communicate with the surface
for the purpose of admitting air, especially the respi-
ratory passages and the whole of the alimentary canal,
the tongue, the inside of the cheeks, the fauces, and the
organ of smell, are covered by a fine epithelium, origi-
nating from the epidermis.^*

177. The texture of the epidermis is extremely simple,
destitute of vessels, nerves, and of true mucous web,
and consequently but little organised; very peculiar,
however,^† remarkably strong, considering its pelluci-
dity and delicacy, so that it resists for a great length of
time maceration, suppuration, and other modes of
[Seite 120] destruction, and reproduced more easily than any other
of the similar parts.

178. It is completely sui generis, somewhat like a
horny lamella, and adheres to the subjacent corium by
the intervention of a mucus, and by numerous very
delicate fibrils which penetrate the latter.^*

The pores which Leuwenhoek imagined in it, do not
exist; but it allows a very ready passage to caloric, car-
bon, hydrogen, and to matters immediately composed
of these, v.c. oil.

179. The importance of the cuticle to organised sys-
tems, is demonstrated by its universality in the animal
and vegetable kingdoms, and by its being distinctly ob-
servable in the embryo from the third month at latest
after conception.

180. The inner part of the cuticle is lined by a fine
mucous membrane, denominated from the opinion of its
discoverer, reticulum Malpighianum, and by means of
which chiefly the cuticle is united more firmly to the
corium.^†

Its nature is mucous, it is very soluble, and, being
thicker in Ethiopians, may be completely separated in
them from both the corium and cuticle, and made to
appear as a true distinct membrane.^‡ (B)

[Seite 121]

181. Our colour resides in it. In all persons the
corium is white, and, in almost all, the cuticle white and
semipellucid, though in Ethiopians it inclines to grey.
But the mucous reticulum varies after birth, with age,
mode of life, and especially with difference of climate.

Thus among the four varieties into which I would
divide the human race, in the first, which may be
termed Caucasian and embraces Europeans (except the
Laplanders and the rest of the Finnish race), the western
Asiatics, and the northern Africans, it is more or less
white.

In the second or Mongolian, including the rest of the
Asiatics (except the Malays of the peninsula beyond the
Ganges), the Finnish races of the north of Europe, as
the Laplanders, &c. and the tribes of Eskimaux diffused
over the north of America, it is yellow or resembling
box wood.

In the third – the Ethiopian, to which the remainder
of the Africans^* belong, it is of a tawny or jet black.

In the fourth or American, comprehending all the
Americans excepting the Eskimaux, it is almost copper
coloured, – of a dark orange or ferruginous hue.

In the fifth or Malaic, in which I include the inhabi-
tants of all the islands in the Pacific Ocean, and of the
Philippine and Sunda, and those of the peninsula of
Malaya, it is more or less tawny, – between the hue of
fresh mahogany and that of cloves or chesnuts.

[Seite 122]

All these shades of colour, as well as the other cha-
racteristics of nations and individuals, run so insensibly
into one another that all division and classification of
them must be more or less arbitrary.

182. The essential cause of the colour of the Mal-
pighian mucus, is, if we mistake not, the proportion of
carbon which is excreted together with hydrogen from the
corium, and in dark nations, being very copious, is pre-
cipitated upon the mucus and combined with it.^*

183. The corium, which is covered by the reticulum
and epidermis, is a membrane, investing the whole body
and defining its surface; tough; very extensible; of dif-
ferent degrees of thickness; every where closely united,
and, as it were, interwoven, with the mucous tela, espe-
cially externally, but more loosely on its internal sur-
face, in which, excepting in certain parts, we generally
discover fat.

184. Besides nerves and absorbents, innumerable
blood vessels, of which we shall speak hereafter, pene-
trate to its external surface, upon which they are shewn,
by minute injection, to form very close and delicate
net-works.

185. A vast number of sebaceous follicles also are
dispersed throughout it, which diffuse over the skin an
oil,^† thin, limpid, and not easily drying,^‡ altogether
[Seite 123] distinct, from the common sweat, and from that which
possesses an odor resembling the odor of goats and
is peculiar to certain parts only.

186. Lastly, almost every part of the corium is beset
with various kinds of hairs,^* chiefly short and delicate,
more or less downy, and found nearly every where but
on the palpebrae, penis, the palms of the hand, and the
soles of the feet. In some parts, they are long and des-
tined for peculiar purposes; such are the capilla-
mentum, the eyebrows, the eye-lashes, the vibrissae,
mustachios, beard, and the hair of the arm-pits and
pudenda.

187. Man is, generally speaking, less hairy than most
other mammalia. But in this respect nations differ.
For, not to mention those nations who to this day care-
fully pluck out their beard or the hair of other parts,
others appear naturally destitute of hair, v.c. the
Tunguses and Burats; (C) on the contrary, creditable
travellers assert that some inhabitants of the islands in
the Pacific and Indian Ocean are remarkable hairy.^†

188. Nor is there less variety in the length, flexibi-
lity, colour, and disposition to curl, both in each race
of men enumerated above (181) and in individuals. V.c.
The hair of the head in the Caucasian variety is rather
dingy or of a nut brown, inclined on one hand to yellow
and on the other to black; in the Mongolian and Ame-
rican, it is black, stiffer, straight, and more sparing; in
the Malay, black, soft, curling, thick, and abundant; in
the Ethiopians, black and woolly: In individuals, espe-
[Seite 124] cially of the Caucasian variety, there are great differ-
ences, and chiefly in connection with temperament, which
is found intimately and invariably connected with the
colour, abundance, disposition to curl, &c. of the hair;^*
and there also exists a remarkable correspondence be-
tween the colour of the hair and of the iris.

189. The direction of the hairs is peculiar in certain
parts, v.c. – spiral on the summit of the head – di-
verging upwards on the pubes – on the exterior of the
arm, as is commonly seen in some anthropomorphous
apes, (v.c. in the satyrus and troglodytes) running in
two opposite directions towards the elbow, i.e. down-
wards from the shoulder, upwards from the wrist; to
say nothing of the eye-lashes and eye-brows.

190. The hairs originate from the inner surface of
the corium, which abounds in fat. They adhere to it
pretty firmly,^† by a curious bulb, consisting of a double
involucrum;^‡ – the exterior vascular and oval, the inte-
rior cylindrical, apparently continuous with the epider-
mis,^§ and sheathing the elastic filaments of which the
hair is composed, and which are generally from five to
ten in each.

191. The hairs are almost incorruptible, and always
anointed by an oily halitus. Of all parts they appear
[Seite 125] most truly electrical. They are very easily nourished,
and even reproduced, unless where the skin is diseased.

192. Besides the functions ascribed to the integu-
ments in the former Section, must be enumerated their
excretory power, by which foreign and injurious matters
are eliminated from the mass of fluids.^*

This is exemplified in the miasmata of exanthematic
diseases, in the smell of the skin after eating garlic,
musk, &c. and in sweating and similar phenomena.

193. What is most worthy our attention, is the tran-
spiration of an aëriform fluid, denominated, after the
very acute philosopher who first applied himself pro-
fessedly to investigate its importance, the perspirabile
Sanctorianum,^† and similar to what is expired from the
lungs.^‡ It likewise is composed of various proportions
of carbon,^§ nitrogen, and hydrogen,^ǁ precipitates lime
from solution, and is unfit to support either flame or
respiration.

194. The sweat, which seldom occurs spontaneously
during health and rest, unless in a high temperature,
appears to arise from the perspirable matter of Sanc-
torius being too much increased in quantity by the
excited action of the cutaneous vessels, and from its
[Seite 126] hydrogen uniting with the oxygen of the atmosphere
and assuming the liquid form.

195. Upon the same hydrogen, variously modified by
the accession of other elements and constituents, would
seem to depend the natural and peculiar odour perceived
in the perspiration and sweat of certain nations and in-
dividuals.^*

196. The quantity of matter perspired from the inte-
guments which, in a well grown adult, are equal to
15 square feet, cannot be accurately estimated, but is
probably about two pounds in 24 hours.^† (D)

---

SECT. XII.
OF THE FUNCTIONS OF THE NERVOUS SYSTEM
IN GENERAL.

[Seite 130]

197. We now come to the other class of functions
termed animal (83, II.), by which the body and mind
are connected. They have obtained their name from
existing in animal systems only, and from enjoying a
greater range than those properly denominated vital.

198. The principal organs of these functions are the
brain, medulla spinalis, and the nerves, the greater part
of which originate from the two former.^* They may
be properly referred to two classes, sensorial and ner-
vous: the former comprehending all excepting the nerves
and their immediate origin, – all that serves more di-
rectly as the connection between the office of the nerves
and the faculties of the mind.

199. Upon this division rests the beautiful observa-
tion of the illustrious Sommerring^† respecting the cor-
respondence between the relative size of each class
with the faculties of the mind. – That the smaller the
nerves are, compared with the sensorial class, the greater
is the developement of the mental faculties. In this
sense, man has the largest brain of all animated beings,
[Seite 131] – if its bulk be compared with that of the nerves arising
from it; but by no means, if its weight be compared
with that of the whole body.

200. Besides the bony cranium, a threefold covering
is afforded to the brain,^* viz. the dura and pia mater,
and, between these two, the tunica arachnoidea.

201. The dura mater,^† which lines the inside of the
cranium, like a periosteum, forms various processes.
By the falx it divides the hemispheres of the cerebrum
and cerebellum; by the tentorium^‡ it supports the
posterior lobes of the cerebrum and prevents their
pressure upon the subjacent cerebellum.

In its various duplicatures it contains and supports
the venous sinuses^§ and prevents their pressure. These
receive the blood returning from the brain to the heart,
the proportion of which to the rest, Zinn long ago very
truly remarked, has been overrated by physiologists.

[Seite 132]

202. Next to the dura mater lies the arachnoid, so
named from its thinness. Its use is not exactly known;
it is destitute of blood-vessels (5), and extended, like
the dura mater, merely over the substance of the brain,
without following the course of its furrows and promi-
nences.

203. On the contrary, the membrane called pia mater
by the ancients, closely follows the cortical substance
of the brain,^* and possesses innumerable blood-vessels
which penetrate into the latter. Hence, if a portion of
this membrane is detached, we find the external surface
very smooth, while the internal is villous and resem-
bles the roots of moss.^† (A)

204. The cerebrum and cerebellum are composed of
various parts which differ in texture and figure, but
the use of which is unknown. The most remark-
able are the four ventricles,^‡ in the two anterior and
fourth of which are found the choroid plexuses, of
whose function also we are ignorant.^§

205. The substance of the brain is twofold: the one
called cineritious or cortical, though not always situated
exteriorly; the other white or medullary. Between the
two, Sömmerring^ǁ has detected a third substance,
most conspicuous in the arbor vitae of the cerebellum
and the posterior lobes of the cerebrum.

[Seite 133]

206. The proportion of the cineritious^* to the cor-
tical substance decreases as age advances, being
greater in children, less in adults. It is almost wholly
composed of very fine vessels, both sanguiferous^† and
colourless (92), of which some few penetrate into the
medullary substance:^‡ the latter is composed, in ad-
dition to these vessels and a fine cellular substance, of
a pultaceous parenchyma, which, if examined with
glasses, exhibits no regular structure,^§ and, upon che-
mical analysis, affords a peculiar matter, in some mea-
sure resembling albumen. (B)

207. The brain, after birth, undergoes a constant and
gentle motion,^ǁ correspondent with respiration; so
that, when the lungs shrink in expiration, the brain
rises a little, but when the chest expands, it again sub-
sides.^**

[Seite 134]

208. The spinal marrow is continuous with the brain,^*
and may be said either to spring from the brain, as
from a root, or, on the contrary, to terminate in it and
grow into its substance.^† Contained in the flexible
canal of the vertebrae, it is enveloped by the same
membranes as the brain: its substance is also twofold,
but the medullary is exterior to the cineritious.

209. From these two sources – the brain and spinal
marrow, arises the greater part of those chords, which
are more or less white and soft, chiefly composed of
cellular canals containing nervous medulla,^‡ and dis-
tributed throughout nearly all the soft parts: some
nerves,^§ however, may be more properly considered as
uniting with the brain and spinal marrow than springing
from them. (C)

[Seite 135]

210. After the numerous experiments^* made by
Haller and other very careful observers, we are cer-
tain, from minute anatomical examination, that many
of the similar parts do not exhibit any true vestige of
nerves; and from surgical observation^† and dissections
of living animals,^‡ that they do not evince the least
sign of feeling.

[Seite 136]

Such are the cellular substance, the epidermis and
reticulum mucosum, the hairs and nails.

The cartilages, bones, periosteum, and marrow.

The tendons, aponeuroses, and ligaments.

Most extended internal membranes, as the dura
mater and arachnoid; the pleura, mediastinum, and
pericardium; the peritonaeum; also the cornea, &c.

The greater part of the absorbent system, especially
the thoracic duct.

Lastly, the secundines and umbilical chord. (D)

211. The ultimate origin of most nerves from the
brain cannot be detected. A question is agitated even
at the present day, – whether the nerves of each side
arise from the corresponding or the opposite portion of
the brain.^* The latter opinion is countenanced by
certain pathological phenomena,^† and by the decussa-
tion of fibres in the medulla oblongata^‡ and conjunc-
tion of the optic nerves.^§ (E)

212. A continuation of the pia mater follows the me-
[Seite 137] dulla of the nerves at their commencement,^* in such a
way, as to unite very delicately with the vascular
cortex.^† But as soon as they have quitted the brain
or medulla spinalis, their structure becomes peculiar,
different from that of all the other similar parts. They
form transverse folds more or less oblique and angular,
long since described by P.P. Mollinelli,^‡ who not in-
aptly compared them to the rugae of earth-worms or
the rings of the aspera arteria.

213. The nerves, especially those which are remark-
able, for instance, the intercostals and par vagum, are
here and there furnished with ganglia, or nodules of a
compact structure and reddish ash colour, but with
whose functions we are scarcely acquainted. I am in-
clined to believe with Zinn^§ that they more intimately
unite the nervous filaments which meet in them from
various directions, so that each fibre passing out
is composed of a portion of every fibre that has en-
tered in.^ǁ

Nearly the same holds good with respect to the plex-
[Seite 138] uses, which are produced by the union and reticulated
anastomoses of different nerves and by a similar con-
texture of filaments into which the nerves are split.

214. The ganglia and plexuses are most abundantly
bestowed upon the spinal nerves and the intercostal
or sympathetic nerve. The latter, united by a few deli-
cate filaments only with the rest of the nervous system,
constitutes a peculiar system, chiefly belonging to the
involuntary functions. For this reason, Bichât, view-
ing it as presiding over organic life, distinguished it
from the other nerves belonging to animal life, to use
his own language.^*

215. The terminations of the nerves are no less con-
cealed from us than their origins. Excepting a few
which spread out in the form of membranes, as the
optic nerve which becomes the retina, and the portio
mollis of the seventh pair which forms a zone in the
spiral lamina of the cochlea, the ultimate filaments of
the rest penetrating into the viscera, muscles, corium,
&c. are so intimately blended with the substance of
these parts as to elude observation.

216. The parts just described, viz. the sensorium and
the nerves originating in it and distributed throughout
the body, constitute that system which, during life, is
the bond of union between the body and the mind.

217. That the mind is closely connected with the
brain, as the material condition of mental phenomena,
is demonstrated, to omit such arguments as the imme-
diate connection between the brain and the organs of
sense, by our consciousness and by the mental disturb-
ances which ensue upon affections of the brain. (F)

[Seite 139]

218. The singular situation and form, before alluded
to, of certain parts of the brain, and likewise some
pathological phenomena, have induced physiologists
to suppose certain parts, in particular, the seat of the
soul. Some have fixed upon the pineal gland,^* others
the corpus callosum,^† the pons Varolii, the medulla
oblongata, the corpora striata, and the water of the
ventricles, which washes against the origin of some
nerves. Others not contented with one spot, have
assigned particular parts of the brain for individual
faculties and propensities. (G)

219. The energy of the whole nervous system does
not depend solely upon the brain. The spinal marrow,
and even the nerves, are possessed of their own powers,
which are sufficient to produce contractions in the
muscles. These powers are probably supported by
the vascular cortex of those parts (212). In man, the
powers proper to the nerves are less, and those depend-
ent upon the brain greater, than in brutes, especially
the cold-blooded.

220. The office of the whole nervous system is two-
fold, – To excite motion in other parts, especially in the
voluntary muscles, of which we shall hereafter speak at
large; and to convey impressions made upon the
organs of sense to the brain, and there to excite per-
[Seite 140] ception or by means of sympathies (56) to give occasion
to reaction.

221. Experiment and observation put these functions
of the nervous system beyond the reach of controversy.
To unfold their nature is difficult indeed. (H)

222. Most opinions on this subject may be divided
into two classes. The one regards the action of the
nervous system as consisting in an oscillatory motion:
The other ascribes it to the motion of a certain fluid,
whose nature is a matter of dispute, by some called
animal spirits^* and supposed to run in vessels, by
others conceived to be a matter analogous to fire, to
light, to a peculiar ether, to oxygen, to electricity, or
to magnetism.

223. Although I would by no means assent to either
of these opinions, I may be allowed to observe, that most
arguments brought by one party against the hypothesis
of the other, must necessarily be rude in proportion to
the subtlety of the oscillations (if such exist) of the
nerves or the nervous fluid.

224. These two hypotheses may, perhaps, be united
by supposing a nervous fluid thrown into oscillatory
vibrations by the action of stimulants.

225. The analogy between the structure of the brain
and some secreting organs, favours the belief of the
existence of a nervous fluid. But tubes and canals are
evidently no more requisite for its conveyance, than
they are requisite in bibulous paper or any other matter
employed for filtering.

The opinion receives much weight from the resem-
[Seite 141] blance of the action of the nerves to the phenomena pro-
duced by the series of a galvanic apparatus and by the
common electrical machine,^* in a living animal or in
parts not quite deprived of vitality. These phenomena
in fact long ago induced some physiologists to compare
the nervous to the electric fluid. The singular and
undeniable effects attributed to animal magnetism,^† as
well as other phenomena which have given rise to the
belief of a kind of sentient atmosphere surrounding
the nerves,^‡ agree very well with the same hypothesis.

226. If we regard the oscillation of the nerves, not
as similar to that which occurs in tense chords, but of
such a description as may be conceived to occur in
the soft pulp of the brain, we shall find many phy-
siological phenomena exactly corresponding with the
supposition.

It is demonstrated that hearing depends upon an
oscillation.

In vision also it probably occurs, although not to
the extent imagined by Euler.

The penetration of Hartley^§ in following up the
conjectures of the Great Newton,^ǁ has rendered it so
probable that the action of the other senses is not very
[Seite 142] dissimilar from this oscillatory motion, that on the
same supposition he very ingeniously explains, prin-
cipally by means of the vapour of the ventricles (called
by him the denser ether),^* first, the association of ideas,
and again, by the assistance of this, most of the func-
tions of the animal faculties. (I)

---

SECT. XIII.
OF THE EXTERNAL SENSES IN GENERAL, AND OF
TOUCH IN PARTICULAR.

[Seite 152]

227. One office of the nerves we found to consist in
communicating to the sensorium the impressions made
by external objects. This is accomplished by the exter-
nal senses, which are, as it were, the watchmen of the
body and the informers of the mind.

The latter alone belong to our present subject. For
to regard, with Gorter, the stimulus which inclines us
to relieve the intestines, the sensation of hunger, and
other internal calls of nature, as so many distinct
senses, is unnecessary minuteness, as Haller long since
observed.^*

228. Touch merits our first attention, because it is
the first to manifest itself, its organ is most extensively
spread over the whole surface, and it is affected by
most properties of external objects.

229. For we perceive not only some qualities, as
heat, hardness, weight, &c. by the touch only, but our
knowledge obtained by other senses respecting some
qualities is rendered more accurate by the touch; such
qualities are figure, distance, &c.

230. It is less fallacious than the rest of the senses,
and by culture capable of such perfection as to supply
the defects of others, particularly of vision.^†

[Seite 153]

231. The skin, whose structure we formerly examined,
is the general organ of touch.^* The immediate seat is
the papillae of the corium, of various forms in diffe-
rent parts, commonly resembling warts,^† in some places
fungous,^‡ in others filamentous.^§ The extremities of
all the cutaneous nerves terminate in these under the
form of pulpy penicilli.

232. The hands are the principal seat of touch pro-
perly so called and regarded as the sense which ex-
amines solidity. The skin of the hands has many
peculiarities. In the palms and on each side of the
joints of the fingers, it is furrowed and free from hairs,
to facilitate the closing of the hand. The extremities
of both fingers and toes are furrowed internally by very
beautiful lines more or less spiral;^ǁ and are shielded
externally by nails.

233. These scutiform nails^** are bestowed upon man
only and a few other genera of mammalia (we allude to
the quadrumana which excel in the sense of touch),^††
[Seite 154] for the purpose of resisting pressure, and thus assisting
the action of the fingers, while examining objects.

They are of a horny nature, but on the whole very
similar to the epidermis. For under them lies the reti-
culum, which in negroes is black;^* and under this is
found the corium, adhering firmly to the periosteum of
the last phalanx. These constituent parts of the nails
are striated lengthwise. The posterior edge, which, in
the hands, is remarkable for a little lunated appear-
ance, is fixed in a furrow of the skin; and the nails
are growing constantly from this, so as to be perfectly
renewed about every six months.

SECT. XIV.
OF TASTE.

[Seite 155]

234. We perceive tastes by the tongue and in some
degree by the other neighbouring internal cutaneous
parts of the mouth, especially by the soft palate, the
fauces, the interior of the cheeks, and lips; by them,
however, we taste only what is acrid and very bitter.^*

235. The chief organ of taste is the tongue,^† agile,
obsequious, changeable in form, and, in its remarkably
fleshy nature, not unlike the heart.

236. Its integuments resemble the skin. They are an
epithelium, performing the office of cuticle; the reti-
culum Malpighianum;^‡ and a papillary membrane,
but little different from the corium.

237. The integuments of the tongue differ from the
skin chiefly in these respects – in the epithelium being
moistened, not by the oily fluid of the skin, but
by a mucus which proceeds from the foramen caecum
of Meibomius^§ and the rest of the glandular expan-
[Seite 156] sion of Morgagni^* – and, secondly, in the conformation
of the papillae, which are commonly divided into petio-
lated, obtuse, and conical.^† The first are very few
in number and situated in a lunated series at the root
of the tongue; the others, of various magnitudes, lie
promiscuously upon the back of the tongue, and chiefly
upon its edges and apex, where taste is most acute.^‡

238. These papillae are furnished with extreme fila-
ments of the lingual branch of the fifth pair;^§ and
through them we probably acquire the power of tasting.

The ninth pair,^ǁ and the branch of the eighth which
also supplies the tongue,^** appear intended rather for
the various movements of this organ, in manducation,
deglutition, speaking, &c.

239. For the tongue to taste properly, it must be
moist, and the substance to be tasted must be liquid,
holding salts in solution.^†† (A) For if either is in a
dry state, we may perceive the presence of the sub-
stances by the common sense of touch, which the tongue
possesses in great acuteness, but cannot discover their
sapid qualities. When the tongue tastes very acutely,
the papillae around its apex and margins are in some
degree erected.

SECT. XV.
OF SMELL.

[Seite 158]

240. While taste and smell are closely related by
the proximity of their organs, they are not less so by
the analogy of their stimuli and by some other circum-
stances. For this reason, they have been generally
named chemical or subjective senses.

By smell we perceive odorous effluvia received by
inspiration and applied principally to that part of the
Schneiderian^* membrane which invests both sides of
the septum narium and the convexities of the turbinated
bones.

241. Although the same mucous membrane lines the
nostrils^† and their sinuses,^‡ its nature appears diffe-
rent in different parts.

Near the external openings it is more similar to the
skin, and beset with sebaceous follicles from which
arise hairs known by the name of vibrissae.

[Seite 159]

On the septum and the turbinated bones it is fungous
and abounds in mucous cryptae.

In the frontal, sphenoidal, ethmoidal, and maxillary
sinuses, it is extremely delicate, and supplied with
an infinite number of blood-vessels which exhale an
aqueous dew.

242. It appears the principal, not to say the sole, use
of the sinuses,^* to supply this watery fluid, which is
perhaps first conveyed to the three meatus of the nos-
trils and afterwards to the other parts of the organ,
preserving them in that constant state of moisture which
is indispensable to the perfection of smell.

The sinuses are so placed, that, in every position of
the head, moisture can pass from one or other of them
into the organ of smell.

243. The principal seat of smell, – the fungous por-
tion of the nasal membrane, besides numerous blood-
vessels, remarkable for being more liable to sponta-
neous hemorrhage than any others in the body, is sup-
plied by nerves, chiefly the first pair,^† which are dis-
tributed on both sides of the septum narium, and also
by two branches of the fifth pair. The former appear
to be the seat of smell:^‡ the latter to serve for the com-
mon feeling of the part, which excites sneezing, &c.

[Seite 160]

244. The extreme filaments of the first pair do not
terminate in papillae, like the nerves of touch and taste,
but, as it were, deliquesce into the spongy and regular
parenchyma of the nasal membrane.

245. The organ of smell is very imperfect and small
at birth. The sinuses scarcely exist. Smell conse-
quently takes place but late – as the internal nostrils
are gradually evolved, and is more acute in proportion
to their size and perfection.^*

246. No external sense is so intimately connected
[Seite 161] with the sensorium and internal senses, nor possesses
such influence over them, as the sense of smell.^*

No other is so liable to idiosyncrasies, nor so power-
ful in exciting and removing syncope.

Nor is any other capable of receiving more de-
licate and delightful impressions; for which reason,
Rousseau very aptly called smell, the sense of imagi-
nation.^†

No sensations can be remembered in so lively a
manner as those which are recalled by peculiar odors.^‡

---

SECT. XVI.
OF HEARING.

[Seite 163]

247. Sound, which is excited by the collision of
elastic bodies and propagated by the air, is perceived
by the sense of hearing,^* and is first received by the
conchiform cartilaginous external ear,^† which few of
our countrymen have the power of moving.^‡ By this
it is collected; then conveyed into the meatus audi-
torius, which is anointed by a bitter cerumen;^§ (A)
and strikes against the membrana tympani, which is
placed obliquely in a circular furrow of the temporal
bone and separates the meatus from the internal ear.

248. Behind this membrane lies the middle portion
of the ear, – the cavity of the tympanum, whose fundus
is directed upwards and inwards.

It contains three^ǁ ossicula auditus: of which the
exterior, or malleus, adheres by its manubrium to the
membrana tympani, is generally united in the adult
[Seite 164] to the circular furrow above mentioned by its spinous
process which is directed forwards, and it lodges its
round head in the body of the incus.

The incus is united to the head of the stapes by the
extremity of its long process which extends into the
cavity of the tympanum.

The stapes, resting its base upon the fenestra ovalis,
runs towards the vestibule of the labyrinth, into which,
sounds, struck against the membrana tympani, are
propagated by the intervention of these three little
bones.

249. The Eustachian tube^* runs from the interior of
the fauces into the cavity of the tympanum: and the
inferior scala of the cochlea has the same direction;
the opening of the latter, termed fenestra rotunda,^† is
closed by a peculiar membrane. The true and prin-
cipal use of each is not sufficiently known.^‡

250. In the deepest part of the petrous bone is placed
the internal ear, consisting of three parts.

First, of the vestibule, placed between the other two,
into which open not only the fenestra ovalis, but the
five orifices of the semicircular canals which lie pos-
teriorly, and the superior scala of the cochlea which is
placed anteriorly.

The vestibule and semicircular canals loosely con-
[Seite 165] tain very delicate membranous bags, lately discovered
by the celebrated Scarpa. Two of these lie in the
vestibule, and three in the semicircular canals.^*

251. They, as well as the cavity of the cochlea, con-
tain a very limpid fluid, bearing the name of Cotunni,
who shewed it to be absorbed by two canals, by him
denominated aqueducts^† and by the no less illustrious
Meckel diverticula;^‡ the one arises from the vestibule,
the other from the inferior scala of the cochlea.

252. The portio mollis of the seventh pair, together
with the portio dura (which afterwards runs along the
Fallopian aqueduct),^§ having entered the internal
acoustic opening, transmits its medullary filaments
into the lower and cribriform part of it.^ǁ These fila-
ments run to the vestibule and semicircular canals, but
especially to the base of the cochlea, where they form
a medullary zonula, marked by beautiful plexiform
striae, which pass between the two laminae of the sep-
tum cochleae.^**

253. The oscillatory tremor, which we formerly fol-
lowed as far as the fenestra ovalis (248), is propagated
to the vestibule, where, by means of the water of
Cotunni (251), it strikes the auditory nerves distributed
among the windings of the labyrinth.

[Seite 166]

254. Besides the muscles of the malleus and
stapes,^* supposed to be voluntary,^† the chorda tym-
pani,^‡ which is placed between the handle of the malleus
and the longer leg of the incus, is believed to moderate
the force of sound that is struck against the membrana
tympani and intended to be propagated along the cavity
of the tympanum.^§ (B)

---

SECT. XVII.
OF SIGHT.

[Seite 168]

255. The instruments of vision, – the eyes,^* are two
moveable globes, fixed to the optic nerves whose de-
cussation we formerly noticed (211), as it were to stalks,
in such a manner, that their insertion is not exactly
opposite the centre of the cornea and iris, but behind
this imaginary axis, – rather nearer to the nose.

256. They consist of various coats containing pellu-
cid humours of different degrees of density, so placed
that the rays of light can pass from the transparent
anterior segment of the bulb to the opposite part of
the fundus.

257. The external coat is called sclerotic. It is de-
ficient in the centre, and that part is filled up by the
cornea, which is transparent, lamellated, more or less
convex, and projects like the segment of a small globe
from one of larger size.^†

258. The interior of the sclerotica is lined by the
chorioid, which abounds in blood-vessels, especially
vorticose veins, and is died on each side by a black
[Seite 169] pigment, adhering, however, but loosely to its concave
surface in the form of mucus.^*

259. The chorioid contains the internal coat – the
retina,^† – a medullary expansion of the optic nerve
which passes through the sclerotica and chorioid,^‡ of
most beautiful texture,^§ and perforated, in the imagi-
nary axis of the eye, between the two principal branches
of the central artery,^ǁ by the singular foramen of
Sömmerring,^** which is surrounded by a yellow
edge.^†† (A)

[Seite 170]

260. The anterior edge of the chorioid is terminated
by a cellular belt, called orbiculus ciliaris, by which it
adheres firmly to a corresponding groove in the scle-
rotic, and from which two other membranes, viz. the
iris and ciliary processes, are expanded in a circular
form.

261. The iris, (whose posterior surface is lined by a
brown pigment and termed uvea) lies anteriorly to the
ciliary processes, is flat, and washed on all sides by
the aqueous humour; narrower towards the nose,
broader towards the temples. Its texture is dense and
cellular and contains no vestige of muscular fibre.
We must regard it, with Zinn,^* as a membrane sui
generis, and not as a propagation from the chorioid.
The anterior surface is differently coloured in different
persons, and, during life, counterfeits a flocculent ap-
pearance.^†

262. The blood-vessels of the iris run chiefly on its
anterior surface, and are continued in the foetus into
the membrana pupillaris,^‡ which begins to open in the
centre at the seventh or eighth month of pregnancy, –
when the eyes have acquired some degree of size, and
when, probably, the elliptic arches of its vessels begin
to be gradually retracted into the inner ring of the iris,
which ring I have never been able to perceive distinctly
before that period.

[Seite 171]

263. The other circular membrane (260) bears the
name of ligamentum or corpus ciliare; and, inclining
backwards, lies at a distance from the iris. Its external
edge is thick^* and adheres to the ciliary circle (260):
the internal is thin and adherent to the margin of the
capsule of the lens. The brown pigment is copiously
diffused over it.

Its anterior surface, lying opposite to the uvea, is
striated. The posterior, lying upon the vitreous hu-
mour, is beautifully separated into about 70 flocculi,
remarkable for an indescribably minute and elegant set
of blood-vessels. These flocculi are named ciliary pro-
cesses and their use is still an object of enquiry.^†

264. In the bulb of the eye, whose coats we have
now described, are contained the humours, of three
principal kinds.

The posterior, and by far the greater, part of the
globe is filled by the vitreous humour, proportionally
larger in the human subject, especially after puberty,
than in other animals, and so dispersed in innumerable
drops throughout the cells of the delicate hyaloid mem-
brane that this membranaceo-lymphatic body has the
singular appearance of a tremulous jelly.

265. Anteriorly it adheres to, and by means of the
zona ciliaris surrounds, the capsule containing the crys-
talline lens, immediately around which lies the water
of Morgagni.

[Seite 172]

The lens itself is very pellucid and cellular, but so
much more dense than the vitreous humour that in the
hand it seems like a very tenacious, although an amaz-
ingly clear, gluten. Its nucleus is more dense than
the exterior lamellae. These may, by management, be
reduced into extremely delicate fibres, converging from
the circumference to the centre.^*

In an adult man the lens is proportionally smaller
than in quadruped mammalia; also less convex, espe-
cially on its posterior surface.

266. The remaining space of the eye is filled by the
aqueous humour, which is very limpid, and divided by
the iris into two chambers: – the anterior and larger sepa-
rating the cornea and iris; and the posterior, in which
the uvea lies towards the corpus ciliare, so small, as
scarcely believed by some to exist.

267. These most valuable parts are defended from
injuries both by the depth of their situation in the orbits
and by the valvular coverings of the eye-lids.

In the duplicature of the palpebrae, lie the sebaceous
follicles of Meibomius,^† thickly distributed; and their
edges are fringed by a triple or quadruple series of
cilia:^‡ the cartilaginous tarsi serve for their support
and expansion, and also facilitate their motion upon
the eye-ball.

Above the eye-lids, to use the language of Cicero,
the skin is covered by the supercilia, which preserve
the eyes from the sweat that flows from the head and
[Seite 173] forehead, and in some measure screen them from too
strong a light.

268. To lubricate the eyes, to preserve their bright-
ness, and to wash away foreign matters, is the office of
the tears. Their chief source is a conglomerate gland
placed in the upper and exterior part of the orbit. It
has numerous but very fine excretory ducts, which are
said to discharge about two ounces of tears upon each
eye during the twenty-four hours: the tears are after-
wards absorbed by the puncta lachrymalia, the function
of which may, in a certain sense, be compared to that of
the lacteals in the villous coat of the small intestines;
from the puncta they are conveyed through the snails’
horns, as they are called, into the lachrymal sac, and
thence pass into the lower meatus of the nostrils.^* (A)

269. Thus much it was necessary to premise upon
the structure of the organ of vision. We now come to
the function of the organ, – to the explanation of vision.

Rays of light falling upon the cornea at an angle
more acute than forty-eight degrees, pass through it,
and, from both its density and figure, are considerably
refracted towards the axis of the eye. On entering the
aqueous humour they experience rather a less degree
of refraction.

Those rays which penetrate the pupil and are received
by the lens, are still more refracted on account of the
greater density of this medium.

The less density of the vitreous humour prevents the
focus of rays from being too small, but allows it to
fall elongated upon the retina and exhibit the image
[Seite 174] of objects, inverse indeed necessarily from the laws
of light.

270. The focus which, in this mode, falls upon the
retina, is considered acute, not absolutely but rela-
tively, on account of the different refrangibility of
colours; but the latitude arising from this aberration
of the rays is so small that it not only does not obscure
the clearness of vision in any perceptible degree, but is
the source of many advantages.^*

271. The celebrated question – why we behold ob-
jects erect, while their image is painted inversely upon
the retina,^† may be easily answered, by considering
that objects are called inverse relatively only to those
which appear erect.

Now, since the images of all objects and of our own
bodies are painted on the retina, each in its relative
situation, this relative situation must correspond as
exactly as if they were viewed erect, so that the mind
(to which a sensation excited by the image and not the
image itself is communicated) is preserved from all
danger of error.

272. Since many conditions are required for distinct
vision, the Creator has wonderfully ordered the func-
tions of these organs.

A sufficient, but, at the same time, a definite, quan-
tity of light, not too intense for distinct vision, is pro-
vided in two modes: – First, according to the greater
or less intensity of the rays, a greater or less number
[Seite 175] of them pass to the lens; – Secondly, that portion
which is superabundant and injurious to vision, is
absorbed.

The first is effected by the motion of the iris; the
second, by the pigmentum nigrum.

273. The iris is endowed with sufficient mobility to
accommodate itself to the intensity and distance of
light, that, when exposed to a strong light or to near
objects it may expand itself and contract the pupil,
but when to a weaker light or more remote objects it
may contract itself and dilate that opening.^* Physio-
logists have given different explanations of this motion.
Some ascribe it to the varied impulse of blood into its
vessels; others to contraction of its imaginary muscular
fibres. I have shewn, in a particular treatise, that
both these circumstances are impossible, and that its
proximate cause may be sought for with more proba-
bility and reason in the vita propria of the iris (42);
the more remote cause, as we formerly hinted (56), can
be solely the reaction of the sensorium.^†

274. The function of the dark pigment, so frequently
mentioned, (258, 261, 263,) viz. to absorb the super-
fluous rays, and its importance to the perfection of
vision, are demonstrated, among other modes, by the
dissection of different kinds of animals, and by the
diseased condition of Albinos, whose eyes are very
tender and impatient of light from the absence of this
pigment.^‡

[Seite 176]

275. The focus of the refracted rays must fall exactly
on the retina, so that the point of vision be neither pro-
duced beyond it nor shortened enough to strike on the
vitreous body.

The latter defect exists in short-sighted persons, from
the too great convexity of the cornea or gibbosity of
the lens.

The former is the defect of long-sighted persons,
in whom there is the opposite conformation of parts.

276. Since a perfect and sound eye beholds near and
remote objects with equal distinctness, it must of neces-
sity be supplied with appropriate powers of accommo-
dation.^* That these internal changes of the eye are
chiefly accomplished by the pressure of the straight
muscles of the ball, I am clearly convinced, from this
among other arguments, – that in the Greenland whale
– an amphibious animal which must see in media of
different densities, nature has most accurately provided
for it, in the remarkable structure and obsequious
flexibility of the sclerotica.^† (C)

277. During the waking state, the eyes are perpe-
tually, although insensibly, agitated, and directed to-
wards the axes of objects, by these muscles.

For, although the whole of the retina is sensible, it
is not all equally calculated to receive the images
of objects.

In the first place, the true axis of the human^‡ eye,
[Seite 177] where the optic nerve enters, is proved, by the well-
known experiment of Mariotte,^* to be nearly insensible
to light. (D) The principal focus of the rest of the retina,
which must be considered as the chief instrument of
distinct vision, falls upon an imaginary axis of the
globe, corresponding with the axis of the cornea and
of the whole eye. This, however, as Kaestner ob-
serves in opposition to Boerhaave, is not to be under-
stood as if only one point of an object could be seen
distinctly at once, while the eye is fixed, and that, to
behold another point, the axis of the eye must be
changed; for the sensation of a complete object is
simple and complete.^†

278. The habit of directing the axes of the eyes
rapidly towards objects is acquired by practice. This
is proved by the example of persons who were born
blind but recovered their sight after puberty;^‡ and
of children, who seldom acquire this facility of motion
before the third month.

279. To habit we must ascribe also the circumstance
of beholding an object singly, although we have two
eyes.^§ For infants at first see double, and the double
vision which occasionally remains after certain dis-
eases of the eyes may be removed by practice and
experience. (E)

280. The combined power of the two eyes does not
[Seite 178] exceed, according to Jurin, that of each, by more than
one thirteenth part.

It is needless to add, what the celebrated painter,
Leonardo da Vinci, long since remarked, – that in
viewing distant objects, it is preferable to employ but
one eye.^* (F)

281. Sight can never occur unless the angle of vision
exceeds 34 seconds. This was proved by the very beau-
tiful experiments of the acute Tob. Mayer, who for-
merly was one of our number. And he demonstrated
the great excellence of the human sight, by shewing
that this still remained the limit of vision under any
light, – under the splendor of the meridian san and the
faint light of a lantern; so that vision remains almost
equally clear although the light be considerably di-
minished.^†

282. We may hence infer the prodigious minuteness
of the images of objects projected upon the retina,^‡
and nevertheless impressed so forcibly upon it, that,
under certain circumstances, their vestiges remain, after
the removal of the objects from before the eye.^§