In the 4th century B.C. medicine emerges as a definite part of the scientific consciousness. During that century there lived and worked one whose thought has stamped itself on the whole sub sequent course of the biological and medical sciences. Aristotle
B.c.) (q.v.) was the great codifier of ancient, science.
The views of Aristotle have had a vast influence in determining the direction of medical thought. For more than 2,000 years Aristotelian philosophy, in more or less corrupted form, constituted the. main irtellectual food of mankind. Without some knowledge of the biological verdicts of Aristotle, it is im possible to understand the course subsequently taken by rational medicine. The influence of Aristotle is specially evident in cer tain basic biological conceptions. (See BIOLOGY : History.) There is one aspect of Aristotelian science, however, to which we must specially refer. Aristotle, following more ancient writers, held that there were four primary and opposite fundamental qualities— the hot and the cold, the wet and the dry. These met in binary combination to constitute the four essences of existences which enter in varying proportions into the constitution of all matter. The four essences, or to give them their usual name, elements, were earth, air, fire and water. Thus, water was wet and cold, fire hot and dry, and so forth. With this theory later writers combined the somewhat similar Hippocratic doctrine which held that the body was composed of the four "humours" or liquids: blood, phlegm, black bile (melancholy), and yellow bile (choler). Excess or defect of one or other humour gave rise to disease. Hence the so-called "humoral pathology" and the doctrine of the "tempera ments" or proportional mixing of the humours which still has its analogues in modern medicine.
Soon after the death of Aristotle in 322 B.C., a great medical school was founded at Alexandria. The two earliest medical teachers at that school were also the greatest —Herophilus of Chalcedon, who flourished about 30o B.0 , and his slightly younger contemporary Erasistratus of Chios, Herophilus may be regarded as the father of anatomy, Erasistratus as the father of physiology.
Herophilus was the first to dissect the human body in public. He recognized the brain as the central organ of the nervous system and the seat of the intelligence. He extended the knowledge of
the parts of the brain, certain of which still bear titles translated from those given by him He was the first to grasp the nature of the nerves, which he distinguished as motor and sensory, though he did not separate them clearly from tendons. He also made the first clear distinction between arteries and veins.
In the Alexandrian period there flourished that view of the structure of the world known as atomic. It was associated with the Epicurean philosophy. (See EPicuRus.) Atomism had reac tions on medicine at Alexandria, where its leading exponent was Erasistratus of Chios.
Erasistratus professed himself a "rationalist," but had nevertheless to invoke the idea of "Nature" as an external power, shaping the ends to which the body acts in order to explain its workings. This is in contrast with Aristotle's view of the "soul" as an entelechy, an innate and inherent factor. (See ARISTOTLE.) To make physiology intelligible, he added a conception, Pneumatism, found also among older thinkers. Pneu matism is the belief that life is associated with a subtle vapour, a pneuma or spirit, which permeates the organism and causes its movements. This vapour is held to have some affinities with the air we breathe. Pneumatism is, in fact, primarily an attempt to explain the phenomena of respiration; it passes on to attempt the explanation of the nature of life and, indeed, of all existence. It is a theory of great historical importance.
Erasistratus observed that every organ is formed of a three fold system of "vessels," veins, arteries and nerves, dividing indefinitely. These, plaited together, he believed, make up the tissues. Blood and two kinds of pneuma are the essential sources of nourishment and movement. The blood is carried by veins. Air is taken in by the lungs and passes to the heart, where it becomes changed into a peculiar pneuma, the vital spirit, which is sent to the various parts of the body by the arteries. It is carried to the brain, and there further changed to a second kind of pneuma, the animal spirit. This in turn is conveyed to the parts of the body by the nerves. It is the prime cause of movement.