THE BIOLOGICAL SCIENCES AND MEDICINE. 803 



contrasted with ancient physiology, he was misled by the natural 

 temptation to carry out, in all its details, a parallel between the ma- 

 chines with which he was familiar, such as clocks and pieces of hy- 

 draulic apparatus, and the living machine. In all such machines there 

 is a central source of power, and the parts of the machine are merely 

 passive distributors of that power. The Cartesian school conceived 

 of the living body as a machine of this kind ; and herein they might 

 have learned from Galen, who, whatever ill use he may have made of 

 the doctrine of " natural faculties," nevertheless had the great merit 

 of perceiving that local forces play a great part in physiology. 



The same truth was recognized by Glisson, but it was first promi- 

 nently brought forward in the Hallerian doctrine of the "vis insita" 

 of muscles. If muscle can contract without nerve, there is an end of 

 the Cartesian mechanical explanation of its contraction by the influx 

 of animal spirits. 



The discoveries of Trembley tended in the same direction. In the 

 fresh-water Hydra no trace was to be found of that complicated ma- 

 chinery upon which the performance of the functions in the higher 

 animals was supposed to depend. And yet the hydra moved, fed, 

 grew, multiplied, and its fragments exhibited all the powers of the 

 whole. And, finally, the work of Caspar F. Wolff,* by demonstrating 

 the fact that the growth and development of both plants and animals 

 take place antecedently to the existence of their grosser organs, and 

 are, in fact, the causes and not the consequences of organization (as 

 then understood), sapped the foundations of the Cartesian physiology 

 as a complete expression of vital phenomena. 



For Wolff, the physical basis of life is a fluid, possessed of a "vis 

 essentialis " and a " solidescibilitas," in virtue of which it gives rise to 

 organization ; and, as he points out, this conclusion strikes at the root 

 of the whole iatro-mechanical system. 



In this country the great authority of John Hunter exerted a simi- 

 lar influence ; though it must be admitted that the two sibylline ut- 

 terances which are the outcome of Hunter's struggles to define his 

 conceptions are often susceptible of more than one interpretation. 

 Nevertheless, on some points Hunter is clear enough. For example, 

 he is of opinion that " spirit is only a property of matter " (" Introduc- 

 tion to Natural History," p. 6), he is prepared to renounce animism 

 {loc. cit., p. 8), and his conception of life is so completely physical 

 that he thinks of it as somethinsf which can exist in a state of combi- 

 nation in the food. " The aliment we take in has in it, in a fixed 

 state, the real life ; and this does not become active until it has got 

 into the lungs ; for there it is freed from its prison " (" Observations 

 on Physiology," p. 113). He also thinks that "it is more in accord 

 with the general principles of the animal machine to suppose that 

 none of its effects are produced from any mechanical principle what- 



* *' Theoria Generationis," 1759. 



