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.
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 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;" 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.
He describes various effects produced on respiration and on the voice by the division of those nerves which are connected with the thorax; and shows particularly the effect of dividing the recurrent branch of his sixth pair of cerebral nerves (the pneumogastric of modern anatomy). He explains how it happens that after division of the spinal cord, provided that division be beneath the lower termination of the neck, the diaphragm will still continue to act—in consequence, namely, of the origin of the phrenic nerve being above the lower termination of the neck.
Before the time of Galen the medical profession was divided into several sects, e.g. Dogmatici, Empirici, Eclectici, Pneumatici, and Episynthetici, who were always disputing with one another. After his time all sects seem to have merged in his followers. The subsequent Greek and Roman biological writers were mere compilers from his works, and as soon as his writings were translated into Arabic they were at once adopted throughout the East to the exclusion of all others. He remained paramount throughout the civilized world until within the last three hundred years. In the records of the College of Physicians of England we read that Dr. Geynes was cited before the college in 1559 for impugning the infallibility of Galen, and was only admitted again into the privileges of his fellowship on acknowledgment of his error, and humble recantation signed with his own hand. Kurt Sprengel has well said that "if the physicians who remained so faithfully attached to Galen's system had inherited his penetrating mind, his observing glance, and his depth, the art of healing would have approached the limit of perfection before all the other sciences; but it was written in the book of destiny that mind and reason were to bend under the yoke of superstition and barbarism, and were only to emerge after centuries of lethargic sleep."
 Hence the name θηρίακαι.
 "De Antidotis," i. 13, vol. xiv. p. 65, Kuhn.
 "Ex ipsius etiam Galeni verbis hanc veritatem confirmari posse, scilicet: non solum posse sanguinem e vena arteriosa in arteriam venosam et inde in sinistrum ventriculum cordis, et postea in arterias transmitti."—"De Motu Cordis," cap. vii.