During the i6th and the first half of the 17th centuries anatomy and physiology had put on their modern dress. Chemical knowledge, however, remained peculiarly backward. Advances had been made in technical processes, but theoretical chemistry was in the hands of the class of dupes who, since the middle ages, had been seeking the Philosopher's Stone. The old theory of the four elements, earth, air, fire and water formed an ill basis for experiment. The great defect of this ancient view of matter was that it contained no definite conception of the nature of a pure substance. The main agent in changing the alchemical to the chemical outlook, was Robert Boyle (1629-91) (q.v.). Under him chemical study was freed from the mystic factor and, more over, loosed from the chains which bound it to medicine, to the disadvantage of both. A fine exponent of this new spirit was John Mayow (q.v.) whose short life prevented him from fulfilling all his early promise. He was the first to recognize clearly that there is in the air a substance or principle concerned at once with combustion, respiration, and the conversion of ven ous into arterial blood. In this sense he was the discoverer of oxygen.
The physical discoveries of the time and the demonstrations of Sanctorius, Harvey and others, gave an impetus to the attempt to explain the animal body on mechanical grounds. One of the earliest and most impressive exponents of physiological theory along these lines was the French philosopher, Rene Descartes (1596-165o) (q.v.). His posthumous book De homine (1662) is the first modern text-book of physiology. Descartes had no ex tensive practical knowledge of the subject. On theoretical grounds he sets forth a very complicated model of animal structure. Subsequent investigation failed to confirm his findings. For a time, however, it attracted many followers. A strong point in his theory is the great stress laid upon the nervous system, and on its power of co-ordinating the different bodily activities.
Just as some explained animal activity on a mechanical basis, so others resorted to chemical interpretation. Of these iatro chemists, the most prominent was Franciscus Sylvius (1614-72)7 noted professor of medicine at Leyden. That progressive uni versity was the seat of the first university laboratory, built at the instigation of Sylvius.
Sylvius devoted much attention to the study of salts and attained to the idea of chemical affinity—an important advance. Well abreast of the anatomical knowledge of his day, and accept ing the broader lines of mechanistic advance in biology, such as the circulation of the blood and the mechanics of muscular mo tion, Sylvius sought to interpret other activities in chemical terms. His position and abilities as a teacher gave his views wide currency and he and his pupils occupy a large part of the field of medical theory until well into the 18th century. By this school almost all forms of vital activity were expressed in terms of "acid and alkali" and of "fermentation." The latter process was assumed to be of a chemical order. The school of Sylvius and its successors added considerably to the knowledge of physi ological processes, notably by examining the digestive fluids, such as saliva, gastric juice, and the secretion of the pancreas.
A third school of medical theorists arose under the German chemist and physician, George Ernest Stahl (166o-1734) (q.v.). His was that view of the nature of the organism which goes under the term vitalism. Though expressed by him in obscure and mystical language, it is, in effect, a return to the Aristotelian position and a denial of the view of Descartes. To Descartes the animal body was a machine. To Stahl the word machine ex pressed exactly what the animal body was not. The phenomena characteristic of the living body are, he considered, not governed by physical and chemical laws, but by laws of a wholly different kind, those of the "sensitive soul." The sensitive soul of Stahl is, in its ultimate analysis, similar to the psyche of Aristotle. Stahl held that the immediate instruments, the natural slaves of this sensitive soul, were chemical processes.
The language and the theories of the iatrophysicists, the iatro chemists and the vitalist of the 17th and 18th centuries, have long been discarded by men of science in their original form. Nevertheless, they represent three attitudes to the activities of living things which have present and current meaning. Each seems to present some aspect of truth. Whether some physiological thinker will combine all three aspects into one coherent whole, it is for the future to decide. In this sense the foundations of modern rational medicine may be said to have been laid by Borelli, Sylvius and Stahl.