young in the ordinary way, by the union of the sexes; in other cases
(_e.g._ flies and fleas) this union of the sexes results in the
production of a _skolex_; while others have no parents, nor do they have
congress--such are the ephemera, tipula, and the like. Aristotle
discusses and rejects the theory that the male reproductive element is
derived from every part of the body. He concludes that "instead of
saying that it comes _from_ all parts of the body, we should say that it
goes _to_ them. It is not the nutrient fluid, but that which is _left
over_, which is secreted. Hence the larger animals have fewer young than
the smaller, for by them the consumption of nutrient material will be
larger and the secretion less. Another point to be noticed is, that the
nutrient fluid is universally distributed through the body, but each
secretion has its separate organ.... It is thus intelligible why
children resemble their parents, since that which makes all the parts of
the body, resembles that which is left over as secretion: thus the
hand, or the face, or the whole animal pre-exists in the sperm, though
in an undifferentiated state ({adioristôs}); and what each of these is
in actuality ({energeia}), such is the sperm in potentiality
({dynamei})."
In later times the two great rival theories put forward to account for
the development of the embryo have been--
(_a_) The theory of Evolution, which makes the embryo pre-existent in
the germ, and only rendered visible by the unfolding and
expansion of its organs.
(_b_) The theory of Epigenesis, which makes the embryo arise, by a
series of successive differentiations, from a simple
homogeneous mass into a complex heterogeneous organism.
The above quotation will show how closely Aristotle held to the theory
of Epigenesis; and in another place he says, "Not at once is the animal
a man or a horse, for the end is last attained; and the specific form is
the end of each development."
Spontaneous generation is nowadays rejected by science; but Aristotle
went so far as to believe that insects, molluscs, and even eels, were
spontaneously generated. It is, however, noteworthy, in view of modern
investigations, that he looked upon _putrefying_ matter as the source of
such development.
A chapter of this work is devoted to the consideration of the hereditary
transmission of peculiarities from parent to offspring.
The fifth and last book contains inquiries into the cause of variation
in the colour of the eyes and hair, the abundance of hair, the sleep of
the embryo, sight and hearing, voice and the teeth.
Widely different opinions have been held from time to time of the value
of Aristotle's biological labours. This philosopher's reputation has,
perhaps, suffered most from those who have praised him most. The praise
has often been of such an exaggerated character as to have become
unmeaning, and to have carried with it the impression of insincerity on
the part of the writer. Such are the laudations of Cuvier. To say as he
does, "Alone, in fact, without predecessors, without having borrowed
anything from the centuries which had gone before, since they had
produced nothing enduring, the disciple of Plato discovered and
demonstrated more truths and executed more scientific labours in a life
of sixty-two years than twenty centuries after him were able to do," is
of course to talk nonsense, for the method which Aristotle applied was
that which Hippocrates had used so well before him; and it is evident to
any one that both his predecessors and contemporaries are frequently
laid under contribution by Aristotle, although the authority is rarely,
if ever, stated by him unless he is about to refute the view put
forward. Exaggerated praise of any author has a tendency to excite
depreciation correspondingly unjust and untrue. It has been so in the
case of this great man. In the endeavour to depose him from the
impossible position to which his panegyrists had exalted him, his
detractors have gone to any length. The principal charges brought
against his biological work have been inaccuracy and hasty
generalization. In support of the charge of inaccuracy, some of the
extraordinary statements which are met with in his works are adduced.
"These," Professor Huxley says, "are not so much to be called errors as
stupidities." Some, however, of the inaccuracies alleged against
Aristotle are fancied rather than real. Thus he is charged with having
represented that the arteries contained nothing but air; that the aorta
arose from the right ventricle; that the heart did not beat in any other
animal but man; that reptiles had no blood, etc.; although in reality he
made no one of these assertions. There remain, nevertheless, the gross
misstatements referred to above, and which really do occur. Such, for
instance, as that there is but a single bone in the neck of the lion;
that there are more teeth in male than in female animals; that the mouth
of the dolphin is placed on the under surface of the body; that the back
of the skull is empty, etc. Although these absurdities undoubtedly occur
in Aristotle's works, it by no means follows that he is responsible for
them. Bearing in mind the curious history of the manuscripts of his
treatises, we shall find it far more reasonable to conclude that such
errors crept in during the process of correction and restoration, by men
apparently ignorant of biology, than that (to take only one case) an
observer who had distinguished the cetacea from fishes and had detected
their hidden mammæ, discovered their lungs, and recognized the distinct
character of their bones, should have been so blind as to fancy that the
mouth of these animals was on the under surface of the body.
That Aristotle made hasty generalizations is true; but it was
unavoidable. Biology was in so early a stage that a theory had often of
necessity to be founded on a very slight basis of facts. Yet,
notwithstanding this drawback, so great was the sagacity of this
philosopher, that many of his generalizations, which he himself probably
looked upon as temporary, have held their ground for twenty centuries,
or, having been lost sight of, have been discovered and put forward as
original by modern biologists. Thus "the advantage of physiological
division of labour was first set forth," says Milne-Edwards, "by myself
in 1827;" and yet Aristotle had said[13] that "whenever Nature is able
to provide two separate instruments for two separate uses, without the
one hampering the other, she does so, instead of acting like a
coppersmith, who for cheapness makes a spit-and-a-candlestick in
one.[14] It is only when this is impossible that she uses one organ for
several functions."
In conclusion, we may say that the great Stagirite expounded the true
principles of science, and that when he failed his failure was caused by
lack of materials. His desire for completeness, perhaps, tempted him at
times to fill in gaps with such makeshifts as came to his hand; but no
one knew better than he did that "theories must be abandoned unless
their teachings tally with the indisputable results of observation."[15]
FOOTNOTES:
[3] Pliny, "Natural History," viii. c. 16.
[4] "History of Animals," i. 11.
[5] Bk. iii. 19.
[6] "Meteorology," iv. 7-11.
[7] "History of Animals," i. 16.
[8] "History of Animals," vi. 3.
[9] "On some of the errors attributed to Aristotle."
[10] "History of Animals," i. 17.
[11] See Professor Huxley's article already referred to.
[12] "Aristotle," by G. H. Lewes, p. 325.
[13] "De Part. Anim.," iv. 6.
[14] {obeliskolychnion}.
[15] "De Gener.," iii. 10, quoted by Dr. Ogle.
GALEN.
_GALEN._
Under the Ptolemies a powerful stimulus was given to biological studies
at Alexandria. Scientific knowledge was carried a step or two beyond the
limit reached by Aristotle. Thus Erasistratus and Herophilus thoroughly
investigated the structure and functions of the valves of the heart, and
were the first to recognize the nerves as organs of sensation. But,
unfortunately, no complete record of the interesting work carried on by
these men has come down to our times. The first writer after Aristotle
whose works arrest attention is Caius Plinius Secundus, whose so-called
"Natural History," in thirty-seven volumes, remains to the present day
as a monument of industrious compilation. But, as a biologist properly
so called, Pliny is absolutely without rank, for he lacked that
practical acquaintance with the subject which alone could enable him to
speak with authority. Of information he had an almost inexhaustible
store; of actual knowledge, the result of observation and experience, so
far as biological studies were concerned, he had but little. This was
largely due to the encyclopædic character of the work he undertook; his
mental powers were weighed down by an enormous mass of unarranged and
ill-digested materials. But it was due also to the peculiar bent of
Pliny's mind. He was not, like Aristotle, an original thinker; he was
essentially a student of books, an immensely industrious but not always
judicious compiler. Often his selections from other works prove that he
failed to appreciate the relative importance of the different subjects
to which he made reference. His knowledge of the Greek language appears,
too, to have been defective, for he gives at times the wrong Latin names
to objects described by his Greek authorities. To these defects must be
added his marvellous readiness to believe any statement, provided only
that it was uncommon; while, on the other hand, he showed an
indefensible scepticism in regard to what was really deserving of
attention. The chief value of his work consists in the historical and
chronological notes of the progress of some of the subjects of which he
treats--fragments of writings which would otherwise be lost to us. Pliny
was killed in the destruction of Pompeii, A.D. 79.
Claudius Galenus was born at Pergamus, in Asia Minor, in the hundred and
thirty-first year of the Christian era. Few writers ever exercised for
so long a time such an undisputed sway over the opinions of mankind as
did this wonderful man. His authority was estimated at a much higher
rate than that of all the biological writers combined who flourished
during a period of more than twelve centuries, and it was often
considered a sufficient argument against a hypothesis, or even an
alleged matter of fact, that it was contrary to Galen.
Endowed by nature with a penetrating genius and a mind of restless
energy, he was eminently qualified to profit by a comprehensive and
liberal education. And such he received. His father, Nicon, an
architect, was a man of learning and ability--a distinguished
mathematician and an astronomer--and seems to have devoted much time and
care to the education of his son. The youth appears to have studied
philosophy successively in the schools of the Stoics, Academics,
Peripatetics, and Epicureans, without attaching himself exclusively to
any one of these, and to have taken from each what he thought to be the
most essential parts of their system, rejecting, however, altogether the
tenets of the Epicureans. At the age of twenty-one, on the death of his
father, he went to Smyrna to continue the study of medicine, to which he
had now devoted himself. After leaving this place and having travelled
extensively, he took up his residence at Alexandria, which was then the
most favourable spot for the pursuit of medical studies. Here he is said
to have remained until he was twenty-eight years of age, when his
reputation secured his appointment, in his native city of Pergamus, to
the office of physician in charge of the athletes in the gymnasia
situated within the precincts of the temple of Æsculapius. For five or
six years he lived in Pergamus, and then a revolt compelled him to leave
his native town. The advantages offered by Rome led him to remove
thither and take up his residence in the capital of the world. Here his
skill, sagacity, and knowledge soon brought him into notice, and excited
the jealousy of the Roman doctors, which was still further increased by
some wonderful cures the young Greek physician succeeded in effecting.
Possibly it was owing to the ill feeling shown to Galen that, on the
outbreak of an epidemic a year afterwards, he left the imperial city and
proceeded to Brindisi, and embarked for Greece. It was his intention to
devote his time to the study of natural history, and for this purpose he
visited Cyprus, Palestine, and Lemnos. While at the last-named place,
however, he was suddenly summoned to Aquileia to meet the Emperors
Marcus Aurelius and Lucius Verus. He travelled through Thrace and
Macedonia on foot, met the imperial personages, and prepared for them a
medicine, for which he seems to have been famous, and which is spoken of
as the _theriac_. It was probably some combination of opium with various
aromatics and stimulants, for antidotes of many different kinds were
habitually taken by the Romans to preserve them from the ill effects of
poison and of the bites of venomous animals.[16]
With the Emperor M. Aurelius he returned to Rome, and became afterwards
doctor to the young Emperor Commodus. He did not, however, remain for a
long period at Rome, and probably passed the greater part of the rest of
his life in his native country.
Although the date of his death is not positively known, yet it appears
from a passage[17] in his writings that he was living in the reign of
Septimius Severus; and Suidas seems to have reason for asserting that he
reached his seventieth year.
Galen's writings represent the common depository of the anatomical
knowledge of the day; what he had learnt from many teachers, rather than
the results of his own personal research. Roughly speaking, they deal
with the following subjects: Anatomy and Physiology, Dietetics and
Hygiene, Pathology, Diagnosis and Semeiology, Pharmacy and Materia
Medica, Therapeutics.
The only works of this voluminous writer at which we can here glance are
those dealing with Anatomy and Physiology. These exhibit numerous
illustrations of Galen's familiarity with practical anatomy, although it
was most likely comparative rather than human anatomy at which he
especially worked. Indeed, he seems to have had but few opportunities of
carrying on human dissections, for he thinks himself happy in having
been able to examine at Alexandria two human skeletons; and he
recommends the dissection of monkeys because of their exact resemblance
to man. To this disadvantage may, perhaps, be attributed the readiness,
which sometimes appears, to assume identity of organization between man
and the brutes. Thus, because in certain animals he found a double
biliary duct, he concluded the same to be the case in man, and in one
instance he proceeded to deduce the cause of disease from this erroneous
assumption.
He supposed that there were three modes of existence in man, namely--
(_a_) The nutritive, which was common to all animals and plants, of
which the liver was the source.
(_b_) The vital, of which the heart was the source.
(_c_) The rational, of which the brain was the source.
Again, he considered that the animal economy possessed four natural
powers--
(1) The attractive.
(2) The alterative or assimilative.
(3) The retentive or digestive.
(4) The expulsive.
Like his predecessors, he asserted that there were four humours, namely,
blood, yellow bile, black bile, and aqueous serum. He held that it was
the office of the liver to complete the process of sanguification
commenced in the stomach, and that during this process the yellow bile
was attracted by the branches of the hepatic duct and gall-bladder; the
black bile being attracted by the spleen, and the aqueous humour by the
two kidneys; while the liver itself retained the pure blood, which was
afterwards attracted by the heart through the vena cava, by whose
ramifications it was distributed to the various parts of the body.
Following Aristotle especially, he regarded hair, nails, arteries,
veins, cartilage, bone, ligament, membranes, glands, fat, and muscle as
the simplest constituents of the body, formed immediately from the
blood, and perfectly homogeneous in character. The organic members,
_e.g._ lungs, liver, etc., he looked upon as formed of several of the
foregoing simple parts.
The osteology contained in Galen's works is nearly as perfect as that of
the present day. He correctly names and describes the bones and sutures
of the cranium; notices the quadrilateral shape of the parietals, the
peculiar situation and shape of the sphenoid, and the form and character
of the ethmoid, malar, maxillary, and nasal bones. He divides the
vertebral columns into cervical, dorsal, and lumbar portions.
With regard to the nervous system, he taught that the nerves of the
senses are distinct from those which impart the power of motion to
muscles--that the former are derived from the anterior parts of the
brain, while the latter arise from the posterior portion, or from the
spinal cord. He maintained that the nerves of the finer senses are
formed of matter too soft to be the vehicles of muscular motion;
whereas, on the other hand, the nerves of motion are too hard to be
susceptible of fine sensibility. His description of the method of
demonstrating the different parts of the brain by dissection is very
interesting, and, like his references to various instruments and
contrivances, proves him to have been a practical and experienced
anatomist.
In his description of the organs and process of nutrition, absorption by
the veins of the stomach is correctly noticed, and the union of the
mesenteric veins into one common _vena portæ_ is pointed out. The
communications between the ramifications of the vena portæ and of the
proper veins of the liver are supposed by Galen to be effected by means
of anastomosing pores or channels. Although it is evident that Galen was
ignorant of the true absorbent system, yet he appears to have been aware
of the _lacteals_; for he says that in addition to those mesenteric
veins which by their union form the vena portæ, there are visible in
every part of the mesentery other veins, proceeding also from the
intestines, which terminate in glands; and he supposes that these veins
are intended for the nourishment of the intestines themselves. Some of
Galen's contemporaries asserted that upon exposing the mesentery of a
sucking animal several small vessels were seen filled _first_ with air,
and _afterwards_ with milk. They had, doubtless, mistaken colourless
lymph for air; but Galen ridicules both assertions, and thereby shows
that he had not examined the contents of the lacteals. This is somewhat
remarkable, because as a rule he omitted no opportunity of determining
with certainty, by vivisection and experiments on living animals, the
uses of the various parts of the body. As an illustration of this, we
have his correct statement, established by experiment, that the pylorus
acts as a valve _only_ during the process of digestion, and that it is
relaxed when digestion is completed.
He recognizes that the flesh of the heart is somewhat different to that
of the muscles of voluntary motion. Its fibres are described as being
arranged in longitudinal and transverse bundles; the former by their
contractions shortening the organ, the latter compressing and narrowing
it. Such statements show that he regarded the heart as essentially
muscular. He thought, however, that it was entirely destitute of nerves.
Although he admitted that possibly it had one small branch derived from
the _nervus vagus_ sent to it, yet he entirely overlooked the great
nervous plexus surrounding the roots of the blood-vessels, from which
branches proceed in company with the branches of the coronary arteries
and veins, and penetrate the muscular substance of the ventricles. He
endeavoured to prove, by experiment, observation, and reasoning, that
the arteries as well as the veins contained blood, and in this
connection he tells an amusing story. A certain teacher of anatomy, who
had declared that the aorta contained no blood, was earnestly desired by
his pupils, who were ardent disciples of Galen, to exhibit the requisite
demonstration, they themselves offering animals for the experiment. He,
however, after various subterfuges, declined, until they promised to
give him a suitable remuneration, which they raised by subscription
among themselves to the amount of a thousand drachmæ (perhaps £30). The
professor, being thus compelled to commence the experiment, totally
failed in his attempt to cut down upon the aorta, to the no small
amusement of his pupils, who, thereupon taking up the experiment
themselves, made an opening into the thorax in the way in which they had
been instructed by Galen, passed one ligature round the aorta at the
part where it attaches itself to the spine, and another at its origin,
and then, by opening the intervening portion of the artery, showed that
blood was contained in it.
The arteries, Galen thought, possessed a pulsative and attractive power
of their own, independently of the heart, the moment of their dilatation
being the moment of their activity. They, in fact, _drew_ their charge
from the heart, as the heart by its diastole _drew_ its charge from the
vena cava and the pulmonary vein. The pulse of the arteries, he also
thought, was propagated by their coats, not by the wave of blood thrown
into them by the heart. He taught that at every systole of the arteries
a certain portion of their contents was discharged at their extremities,
namely, by the exhalents and secretory vessels. Though he demonstrated
the anastomosis of arteries and veins, he nowhere hints his belief that
the contents of the former pass into the latter, to be conveyed back to
the heart, and from it to be again diffused over the body. He made a
near approach to the Harveian theory of the circulation, as Harvey
himself admits in his "De Motu Cordis;"[18] but the grand point of
difference between Galen and Harvey is the question whether or not, at
every systole of the left ventricle, more blood is thrown out than is
expended on exhalation, secretion, and nutrition. Upon this point Galen
held the negative, and Harvey, as we all know, the affirmative.
The famous Asclepiads held that respiration was for the generation of
the soul itself, breath and life being thus considered to be identical.
Hippocrates thought it was for the nutrition and refrigeration of the
innate heat, Aristotle for its ventilation, Erasistratus for the
filling of the arteries with spirits. All these opinions are discussed
and commented upon by Galen, who determines the purposes of respiration
to be (1) to preserve the animal heat; (2) to evacuate from the blood
the products of combustion.
He conjectured that there was in atmospheric air not only a quality
friendly to the vital spirit, but also a quality inimical to it, which
conjecture he drew from observation of the various phenomena
accompanying the support and the extinction of flame; and he says that
if we could find out why flame is extinguished by absence of the air, we
might then know the nature of that substance which imparts warmth to the
blood during the process of respiration.
On another occasion he says that it is evidently the _quality_ and not
the _quantity_ of the air which is necessary to life. He further shows
that he recognized the analogy between respiration and combustion, by
comparing the lungs to a lamp, the heart to its wick, the blood to the
oil, and the animal heat to the flame.
From certain observations in various parts of his works, it appears
that, although ignorant of the doctrine of atmospheric pressure, he was
acquainted with some of its practical effects. Thus, he says, if you put
one end of an open tube under water and suck out the air with the other
end, you will draw up water into the mouth, and that it is in this way
that infants extract the milk from the mother's breast.
Again, Erasistratus supposed that the vapour of charcoal and of certain
pits and wells was fatal to life because _lighter_ than common air, but
Galen maintained it to be _heavier_.
He describes two kinds of respiration, one by the mouths of the arteries
of the lungs, and one by the mouths of the arteries of the skin. In each
case, he says, the surrounding air is drawn into the vessels during
their diastole, for the purpose of cooling the blood, and during their
systole the fuliginous particles derived from the blood and other fluids
of the body are forced out.
He considers the diaphragm to be the principal muscle of respiration,
but he makes a clear distinction between ordinary respiration, which he
calls a natural and involuntary effort, and that deliberate and forced
respiration which is obedient to the will; and he says that there are
different muscles for the two purposes. Elsewhere he particularly points
out the two sets of intercostal muscles and their mode of action, of
which, before his time, he asserts that anatomists were ignorant.