to the cavity of the pericardium. For a part of this cavity (_sinus
transversus pericardii_) lies between the aorta, on the one hand, and
the pulmonary vessels with the bifurcation of the trachea, on the other
hand, and is much more conspicuous in some animals than in man. It is
strictly correct, therefore, in Aristotle’s words, to say that where
the heart and the windpipe are connected “it is hollow.” If he had
meant to speak of one of the pulmonary veins, or of any of the cavities
of the heart, he would have used the terms πόροι or κοιλίας which he
always employs for these parts.
According to Aristotle, then, the air taken into the lungs passes, from
the final ramifications of the bronchial tubes into the corresponding
branches of the pulmonary blood-vessels, not through openings, but by
transudation, or, as we should nowadays say, diffusion, through the
thin partitions formed by the applied coats of the two sets of canals.
But the “pneuma” which thus reached the interior of the blood-vessels
was not, in Aristotle’s opinion, exactly the same thing as the air.
It was “ἀὴρ πολὺς ῥέων καὶ ἀθρόος” (“De Mundo,” iv. 9)—subtilised and
condensed air; and it is hard to make out whether Aristotle considered
it to possess the physical properties of an elastic fluid or those of
a liquid. As he affirms that all the cavities of the heart contain
blood (_F_), it is clear that he did not hold the erroneous view
propounded in the next generation by Erasistratus. On the other hand,
the fact that he supposes that the spermatic arteries do not contain
blood but only an αἱματῶδης ὑγρόν (“Hist. Animalium,” iii. 1), shows
that his notions respecting the contents of the arteries were vague.
Nor does he seem to have known that the pulse is characteristic only
of the arteries; and as he thought that the arteries end in solid
fibrous bands, he naturally could not have entertained the faintest
conception of the true motion of the blood. But, without attempting
to read into Aristotle modern conceptions which never entered his
mind, it is only just to observe that his view of what becomes of the
air taken into the lungs is by no means worthy of contempt as a gross
error. On the contrary, here, as in the case of his anatomy of the
heart, what Aristotle asserts is true as far as it goes. Something
does actually pass from the air contained in the lungs through the
coats of the vessels into the blood, and thence to the heart; to wit,
oxygen. And I think that it speaks very well for ancient Greek science
that the investigator of so difficult a physiological problem as that
of respiration, should have arrived at a conclusion, the statement of
which, after the lapse of more than two thousand years, can be accepted
as a thoroughly established scientific truth.
I trust that the case in favour of removing the statements about
the heart, from the list of the “errors of Aristotle” is now clear;
and that the evidence proves, on the contrary, that they justify
us in forming a very favourable estimate of the oldest anatomical
investigations among the Greeks of which any sufficient record remains.
But is Aristotle to be credited with the merit of having ascertained so
much of the truth? This question will not appear superfluous to those
who are acquainted with the extraordinary history of Aristotle’s works,
or who adopt the conclusion of Aubert and Wimmer, that, of the ten
books of the “Historia Animalium” which have come down to us, three
are largely or entirely spurious, and that the others contain many
interpolations by later writers.
It so happens, however, that, apart from other reasons, there are
satisfactory internal grounds for ascribing the account of the heart
to a writer of the time at which Aristotle lived. For, within thirty
years of his death, the anatomists of the Alexandrian school had
thoroughly investigated the structure and the functions of the valves
of the heart. During this time, the manuscripts of Aristotle were in
the possession of Theophrastus; and no interpolator of later date would
have shown that he was ignorant of the nature and significance of
these important structures, by the brief and obscure allusion—“in its
cavities there are tendons” (_A_). On the other hand, Polybus, whose
account of the vascular system is quoted in the “Historia Animalium”
was an elder contemporary of Aristotle. Hence, if any part of the
work faithfully represents that which Aristotle taught, we may safely
conclude that the description of the heart does so. Having granted
this much, however, it is another question, whether Aristotle is to
be regarded as the first discoverer of the facts which he has so well
stated, or whether he, like other men, was the intellectual child of
his time and simply carried on a step or two the work which had been
commenced by others.
On the subject of Aristotle’s significance as an original worker in
biology extraordinarily divergent views have been put forward. If we
are to adopt Cuvier’s estimate, Aristotle was simply a miracle:—
“Avant Aristote la philosophie, entièrement spéculative,
se perdait dans les abstractions dépourvues de fondement;
la science n’existait pas. Il semble qu’elle soit
sortie toute faite du cerveau d’Aristote comme Minerve,
toute armée, du cerveau de Jupiter. Seul, en effet,
sans antécédents, sans rien emprunter aux siècles qui
l’avaient précédé, puisqu’ils n’avaient rien produit de
solide, le disciple de Platon découvrit et démontra plus
de vérités, executa plus de travaux scientifiques en un
vie de soixante-deux ans, qu’après lui vingt siècles n’en
ont pu faire,”[39] etc. etc.
“Aristote est le premier qui ait introduit la méthode
de l’induction, de la comparaison des observations
pour en faire sortir des idées générales, et celle de
l’expérience pour multiplier les faits dont ces idées
générales peuvent être déduites.”—ii. p. 515.
The late Mr. G. H. Lewes,[40] on the contrary, tells us “on a
superficial examination, therefore, he [Aristotle] will seem to have
given tolerable descriptions; especially if approached with that
disposition to discover marvels which unconsciously determines us
in our study of eminent writers. But a more unbiassed and impartial
criticism will disclose that he has given no single anatomical
description of the least value. All that he knew may have been known,
and probably was known, without dissection.... I do not assert that
he never opened an animal; on the contrary it seems highly probable
that he had opened many.... He never followed the course of a vessel
or a nerve; never laid bare the origin and insertion of a muscle;
never discriminated the component parts of organs; never made clear to
himself the connection of organs into systems.”—(pp. 156-7.)
In the face of the description of the heart and lungs, just quoted,
I think we may venture to say that no one who has acquired even an
elementary practical acquaintance with anatomy, and knows of his own
knowledge that which Aristotle describes, will agree with the opinion
expressed by Mr. Lewes; and those who turn to the accounts of the
structure of the rock lobster and the common lobster, or to that of the
Cephalopods and other Mollusks, in the fourth book of the “Historia
Animalium” will probably feel inclined to object to it still more
strongly.
On the other hand, Cuvier’s exaggerated panegyric will as little bear
the test of cool discussion. In Greece, the century before Aristotle’s
birth was a period of great intellectual activity, in the field of
physical science no less than elsewhere. The method of induction has
never been used to better effect than by Hippocrates; and the labours
of such men as Alkmeon, Demokritus, and Polybus, among Aristotle’s
predecessors; Diokles, and Praxagoras, among his contemporaries, laid a
solid foundation for the scientific study of anatomy and development,
independently of his labours. Aristotle himself informs us that the
dissection of animals was commonly practised; that the aorta had been
distinguished from the great vein; and that the connection of both with
the heart had been observed by his predecessors. What they thought
about the structure of the heart itself or that of the lungs, he does
not tell us, and we have no means of knowing. So far from arrogantly
suggesting that he owed nothing to his predecessors, Aristotle is
careful to refer to their observations, and to explain why, in his
judgment, they fell into the errors which he corrects.
Aristotle’s knowledge, in fact, appears to have stood in the same
relation to that of such men as Polybus and Diogenes of Apollonia, as
that of Herophilus and Erasistratus did to his own, so far as the heart
is concerned. He carried science a step beyond the point at which he
found it; a meritorious, but not a miraculous, achievement. What he
did, required the possession of very good powers of observation; if
they had been powers of the highest class, he could hardly have left
such conspicuous objects as the valves of the heart to be discovered by
his successors.
And this leads me to make a final remark upon a singular feature of
the “Historia Animalium.” As a whole, it is a most notable production,
full of accurate information, and of extremely acute generalisations of
the observations accumulated by naturalists up to that time. And yet,
every here and there, one stumbles upon assertions respecting matters
which lie within the scope of the commonest inspection, which are not
so much to be called errors, as stupidities. What is to be made of the
statement that the sutures of women’s skulls are different from those
of men; that men and sundry male animals have more teeth than their
respective females; that the back of the skull is empty; and so on? It
is simply incredible to me, that the Aristotle who wrote the account of
the heart, also committed himself to absurdities which can be excused
by no theoretical prepossession and which are contradicted by the
plainest observation.
What, after all, were the original manuscripts of the “Historia
Animalium”? If they were notes of Aristotle’s lectures taken by some of
his students, any lecturer who has chanced to look through such notes,
would find the interspersion of a foundation of general and sometimes
minute accuracy, with patches of transcendent blundering, perfectly
intelligible. Some competent Greek scholar may perhaps think it worth
while to tell us what may be said for or against the hypothesis thus
hinted. One obvious difficulty in the way of adopting it is the fact
that, in other works, Aristotle refers to the “Historia Animalium” as
if it had already been made public by himself.
IX.
ON THE HYPOTHESIS THAT ANIMALS ARE AUTOMATA, AND ITS HISTORY.
The first half of the seventeenth century is one of the great epochs
of biological science. For though suggestions and indications of the
conceptions which took definite shape, at that time, are to be met
with in works of earlier date, they are little more than the shadows
which coming truth casts forward; men’s knowledge was neither extensive
enough, nor exact enough, to show them the solid body of fact which
threw these shadows.
But, in the seventeenth century, the idea that the physical processes
of life are capable of being explained in the same way as other
physical phenomena, and, therefore, that the living body is a
mechanism, was proved to be true for certain classes of vital actions;
and, having thus taken firm root in irrefragable fact, this conception
has not only successfully repelled every assault which has been made
upon it, but has steadily grown in force and extent of application,
until it is now the expressed or implied fundamental proposition of the
whole doctrine of scientific Physiology.
If we ask to whom mankind are indebted for this great service, the
general voice will name William Harvey. For, by his discovery of the
circulation of the blood in the higher animals, by his explanation of
the nature of the mechanism by which that circulation is effected,
and by his no less remarkable, though less known, investigations of
the process of development, Harvey solidly laid the foundations of
all those physical explanations of the functions of sustentation and
reproduction which modern physiologists have achieved.
But the living body is not only sustained and reproduced: it adjusts
itself to external and internal changes; it moves and feels. The
attempt to reduce the endless complexities of animal motion and
feeling to law and order is, at least, as important a part of the task
of the physiologist as the elucidation of what are sometimes called
the vegetative processes. Harvey did not make this attempt himself;
but the influence of his work upon the man who did make it is patent
and unquestionable. This man was René Descartes, who, though by many
years Harvey’s junior, died before him; and yet, in his short span
of fifty-four years, took an undisputed place, not only among the
chiefs of philosophy, but amongst the greatest and most original of
mathematicians; while, in my belief, he is no less certainly entitled
to the rank of a great and original physiologist; inasmuch as he did
for the physiology of motion and sensation that which Harvey had done
for the circulation of the blood, and opened up that road to the
mechanical theory of these processes, which has been followed by all
his successors.
Descartes was no mere speculator, as some would have us believe: but a
man who knew of his own knowledge what was to be known of the facts of
anatomy and physiology in his day. He was an unwearied dissector and
observer; and it is said, that, on a visitor once asking to see his
library, Descartes led him into a room set aside for dissections, and
full of specimens under examination. “There,” said he, “is my library.”
I anticipate a smile of incredulity when I thus champion Descartes’
claim to be considered a physiologist of the first rank. I expect to
be told that I have read into his works what I find there, and to be
asked, Why is it that we are left to discover Descartes’ deserts at
this time of day, more than two centuries after his death? How is it
that Descartes is utterly ignored in some of the latest works which
treat expressly of the subject in which he is said to have been so
great?
It is much easier to ask such questions than to answer them, especially
if one desires to be on good terms with one’s contemporaries; but, if I
must give an answer, it is this: The growth of physical science is now
so prodigiously rapid, that those who are actively engaged in keeping
up with the present, have much ado to find time to look at the past,
and even grow into the habit of neglecting it. But, natural as this
result may be, it is none the less detrimental. The intellect loses,
for there is assuredly no more effectual method of clearing up one’s
own mind on any subject than by talking it over, so to speak, with
men of real power and grasp, who have considered it from a totally
different point of view. The parallax of time helps us to the true
position of a conception, as the parallax of space helps us to that of
a star. And the moral nature loses no less. It is well to turn aside
from the fretful stir of the present and to dwell with gratitude and
respect upon the services of those “mighty men of old who have gone
down to the grave with their weapons of war,” but who, while they yet
lived, won splendid victories over ignorance. It is well, again, to
reflect that the fame of Descartes filled all Europe, and his authority
overshadowed it, for a century; while now, most of those who know his
name think of him, either as a person who had some preposterous notions
about vortices and was deservedly annihilated by the great Sir Isaac
Newton; or as the apostle of an essentially vicious method of deductive
speculation; and that, nevertheless, neither the chatter of shifting
opinion, nor the silence of personal oblivion, has in the slightest
degree affected the growth of the great ideas of which he was the
instrument and the mouthpiece.
It is a matter of fact that the greatest physiologist of the eighteenth
century, Haller, in treating of the functions of nerve, does little
more than reproduce and enlarge upon the ideas of Descartes. It is a
matter of fact that David Hartley, in his remarkable work the “Essay
on Man,” expressly, though still insufficiently, acknowledges the
resemblance of his fundamental conceptions to those of Descartes; and
I shall now endeavour to show that a series of propositions, which
constitute the foundation and essence of the modern physiology of the
nervous system, are fully expressed and illustrated in the works of
Descartes.
I. _The brain is the organ of sensation, thought, and
emotion; that is to say, some change in the condition of
the matter of this organ is the invariable antecedent of
the state of consciousness to which each of these terms
is applied._
In the “Principes de la Philosophie” (§ 169), Descartes says:—[41]
“Although the soul is united to the whole body, its
principal functions are, nevertheless, performed in
the brain; it is here that it not only understands and
imagines, but also feels; and this is effected by the
intermediation of the nerves, which extend in the form
of delicate threads from the brain to all parts of the
body, to which they are attached in such a manner, that
we can hardly touch any part of the body without setting
the extremity of some nerve in motion. This motion passes
along the nerve to that part of the brain which is the
common sensorium, as I have sufficiently explained in
my Treatise on Dioptrics; and the movements which thus
travel along the nerves, as far as that part of the
brain with which the soul is closely joined and united,
cause it, by reason of their diverse characters, to have
different thoughts. And it is these different thoughts of
the soul, which arise immediately from the movements that
are excited by the nerves in the brain, which we properly
term our feelings, or the perceptions of our senses.”
Elsewhere,[42] Descartes, in arguing that the seat of the passions is
not (as many suppose) the heart, but the brain, uses the following
remarkable language:—
“The opinion of those who think that the soul receives
its passions in the heart, is of no weight, for it is
based upon the fact that the passions cause a change to
be felt in that organ; and it is easy to see that this
change is felt, as if it were in the heart, only by the
intermediation of a little nerve which descends from the
brain to it; Just as pain is felt, as if it were in the
foot, by the intermediation of the nerves of the foot;
and the stars are perceived, as if they were in the
heavens, by the intermediation of their light and of the
optic nerves. So that it is no more necessary for the
soul to exert its functions immediately in the heart, to
feel its passions there, than it is necessary that it
should be in the heavens to see the stars there.”
This definite allocation of all the phenomena of consciousness to the
brain as their organ, was a step the value of which it is difficult
for us to appraise, so completely has Descartes’ view incorporated
itself with every-day thought and common language. A lunatic is said
to be “crack-brained” or “touched in the head,” a confused thinker
is “muddle-headed,” while a clever man is said to have “plenty of
brains;” but it must be remembered that at the end of the last century
a considerable, though much over-estimated, anatomist, Bichat, so far
from having reached the level of Descartes, could gravely argue that
the apparatuses of organic life are the sole seat of the passions,
which in no way affect the brain, except so far as it is the agent by
which the influence of the passions is transmitted to the muscles.[43]
Modern physiology, aided by pathology, easily demonstrates that the
brain is the seat of all forms of consciousness, and fully bears out
Descartes’ explanation of the reference of those sensations in the
viscera which accompany intense emotion, to these organs. It proves,
directly, that those states of consciousness which we call sensations
are the immediate consequent of a change in the brain excited by
the sensory nerves; and, on the well-known effects of injuries, of
stimulants, and of narcotics, it bases the conclusion that thought and
emotion are, in like manner, the consequents of physical antecedents.
II. _The movements of animals are due to the change of
form of muscles, which shorten and become thicker; and
this change of form in a muscle arises from a motion of
the substance contained within the nerves which go to the
muscle_.
In the “Passions de l’Âme,” Art. vii., Descartes writes:—
“Moreover, we know that all the movements of the limbs
depend on the muscles, and that these muscles are opposed
to one another in such a manner, that when one of them
shortens, it draws along the part of the body to which
it is attached, and so gives rise to a simultaneous
elongation of the muscle which is opposed to it. Then,
if it happens, afterwards, that the latter shortens, it
causes the former to elongate, and draws towards itself
the part to which it is attached. Lastly, we know that
all these movements of the muscles, as all the senses,
depend on the nerves, which are like little threads or
tubes, which all come from the brain, and, like it,
contain a certain very subtle air or wind, termed the
animal spirits.”
The property of muscle mentioned by Descartes now goes by the general
name of contractility, but his definition of it remains untouched. The
long-continued controversy whether contractile substance, speaking
generally, has an inherent power of contraction, or whether it
contracts only in virtue of an influence exerted by nerve, is now
settled in Haller’s favour; but Descartes’ statement of the dependence
of muscular contraction on nerve holds good for the higher forms of
muscle, under normal circumstances; so that, although the structure of
the various modifications of contractile matter has been worked out
with astonishing minuteness—although the delicate physical and chemical
changes which accompany muscular contraction have been determined to
an extent of which Descartes could not have dreamed, and have quite
upset his hypothesis that the cause of the shortening and thickening of
the muscle is the flow of animal spirits into it from the nerves—the
important and fundamental part of his statement remains perfectly true.
The like may be affirmed of what he says about nerve. We know now that
nerves are not exactly tubes, and that “animal spirits” are myths; but
the exquisitely refined methods of investigation of Dubois-Reymond and
of Helmholz have no less clearly proved that the antecedent of ordinary
muscular contraction is a motion of the molecules of the nerve going to
the muscle; and that this motion is propagated with a measurable, and
by no means great, velocity, through the substance of the nerve towards
the muscle.
With the progress of research, the term “animal spirits” gave way to
“nervous fluid,” and “nervous fluid” has now given way to “molecular
motion of nerve-substance.” Our conceptions of what takes place in
nerve have altered in the same way as our conceptions of what takes
place in a conducting wire have altered, since electricity was shown to
be not a fluid, but a mode of molecular motion. The change is of vast
importance, but it does not affect Descartes’ fundamental idea, that a
change in the substance of a motor nerve propagated towards a muscle is
the ordinary cause of muscular contraction.
III. _The sensations of animals are due to a motion of
the substance of the nerves which connect the sensory
organs with the brain._
In _La Dioptrique_ (Discours Quatrième), Descartes explains, more fully
than in the passage cited above, his hypothesis of the mode of action
of sensory nerves:—