Although he has written at some length on generation, he expresses much uncertainty about the use and im portance of the testes. At one time he asserts, that they prepare the seminal fluid by a vis insita ; at ano ther, he seems to think that they are organs rather of convenience than of essential utility ; that nature de signed them chiefly as a counterpoise, to prevent the retraction of the spermatic chord ; that some animals have been able to procreate after castration, when the spermatic chord was not retracted; and that serpents and fishes, which, in his opinion, have no testes, are abundantly prolific. His vital principle, like that of Hippocrates, was fire, of the same nature with the pure element of the stars. This fire resided, prior to con ception, in the seminal fluid of the male ; and, though he speaks with great hesitation on the subject, he seems to think, that, with the assistance of what he calls the for mal cause, not unlike the /hes; of Plato, it constructed the body, and thus formed for itself a habitation. Yet, notwithstanding his errors, Aristotle did much both for anatomy and natural history; and science is particularly indebted to him, not only for the important facts which he brought to light, but for the ardour which he inspired for similar inquiries.
The path which Aristotle had thus marked out, was pursued with equal enthusiasm by Diocles of Carystus, who wrote a treatise on the method of dissecting bo dies; and by Praxagoras of Cos, who restricted the meaning of vein and artery to the organs known at pre sent by these names. This restriction was of consider able importance, as these words had, before that time, been very vaguely applied. Hippocrates had given the name of vein to the optic nerve, and the ureters and the arteries were included with the ligaments and tendons under the general appellation of nerves,—a term which, in Hippocrates, Plato, and Aristotle, had little connec tion with its present meaning.
Among the promoters of the science of anatomy, it would be unfair to omit Alexander the Great, at whose particular request Aristotle undertook the History of Animals, and at whose expense he was furnished with the means of prosecuting his inquiries. The views of Alexander were eagerly adopted by the Ptolemies, his successors in Egypt; through whose royal patronage anatomists first enjoyed the opportunity of examining the structure of the human body. Without such protec tion, no private individual would have attempted the dissection of a human subject : against which, the pre judices of the Greeks were not less violent than those of the Jews and Egyptians.
The lirst and principal anatomist employed by the Ptolemies was I lei opnilus of Chalcedon, the disciple of Praxagoras. his attention was chiefly directed to the nervous system ; and, in the course of his researches, he made a discovery which constituted a new .,era in
the history of anatomy. Before his time, the brain and its functions were very imperfectly understood. Ifippo crates had mentioned its two membranes; Aristotle its division into halves, a cavity in its middle, and a smaller brain in the back of the head. Herophilus, examining it with more attention, discovered no less than four ca vities, traced several of its nerves, and describes a va riety of other parts, sonic of which are distinguished by his name. He was the first, in short, who ascertained the functions of the brain, the spinal marrow, and the nerves. So minute was the attention which he paid to the blood-vessels, that he characterised the veins and the arteries by the thickness of their coats; and was the first Greek physician who wrote an accurate treatise on the pulse ; though, to form any prognosis from its beat ings, he tells us, that it is necessary to be first acquaint ed with geometry and music. Herophilus is considered as the founder of the medical school at Alexandria, and the first anatomist who taught osteology from the hu man skeleton.
Contemporary with Herophilus was Erasistratus, the grandson of Aristotle, and the pupil of Chrysippus. Erasistratus was one of the principal physicians at the court of Antiochus in Syria ; and he likewise enjoyed frequent permission to examine the hodies of criminals. He was well acquainted with the functions of the brain ; tells us, that its different cavities communicate ; and mentions the distribution of the nerves, which he consi ders as the primary organs of sense and motion. It is allowed by Galen, that the valves of the heart, and those which are placed at the commencement of the great ar teries, were first discovered by Erasistratus. If Galen, however, be correct, the treatise TrEci .a.p3s,as, found in the works of Hippocrates, in which these organs are mentioned, cannot be genuine. Erasistratus was the first who asserted, that digestion is performed by the action of the stomach ; and the next after llerophilus who traced the vessels which convey the chyle from the intestines: he maintained, that the blood is distributed through the liver for the secretion of the bile ; and con futed an opinion which Plato had adopted from mistaking a passage in Hippocrates,—that our drink passes through the windpipe into the lungs. In a fragment of his works preserved by Galen, from whom we derive nearly all the information which we possess concerning him and Herophilus, he speaks of a happy disposition of the muscles for the movement:of the limbs. From this it may be inferred, that the function of the muscles was then known; and, as neither he nor Herophilus have claimed the discovery, it was probably known before their time.