100 THE PHILOSOPHY OF BIOLOGY 



Irritability and contractility are general pro- 

 perties of the organism. These properties are illus- 

 trated by the irritability of an Amceba or Paramcecium 

 to stimuli of many kinds ; by the movements of the 

 pseudopodia of the former animal, or of the cilia of 

 the latter ; by the nervous irritability of the higher 

 animal, and the contraction of its muscles when they 

 are stimulated. They are among the fundamental 

 properties or functions of living protoplasm, and their 

 study is of paramount interest, and carries us to the 

 very centre of the problem of the activities of the 

 organism. Naturally physiologists have never ceased 

 to attempt to describe irritability and contractility in 

 terms of physics, but though we may be quite certain 

 that the things that do occur in these phenomena are 

 controlled physico-chemical reactions, it must be re- 

 membered that what we positively know about their 

 precise nature is exceedingly little. 



What is the nature of a nervous impulse ? When 

 a receptor organ is stimulated, as, for instance, when 

 light impinges on the cone cells of the retina, or when 

 the nerve-endings in a ' heat-spot ' in the skin are 

 warmed, or when the wires conveying an electric 

 current are laid on a naked nerve, an impulse is set 

 up in the nerve proceeding from the place stimulated, 

 and we must suppose that approximately the same 

 amount of energy moves along the nerve as was com- 

 municated to the receptor or the nerve itself by a 

 stimulus of minimal strength. How does it so move ? 



suppose now that our consciousness, something which has nothing to do, it 

 must be noted, with energy changes in the body, can react on the body. If 

 we show a dog an attractive bone it will secrete saliva ; if we show it again 

 and again, the same thing occurs. But after certain such trials the dog will 

 realise that he is being played with, and the exhibition of the bone no longer 

 evokes a flow of secretion. Why is this ? The whole process has now become 

 more mysterious than ever. 



