CHAPTER IV. 

 THE VITAL PBOPERTIES OF THE CELL (continued). 



Phenomena of Stimulation. The most remarkable pro- 

 perty of protoplasm is its power of reacting to stimuli : its 

 Irritability. 1 



By this is understood, as Sachs (IV. 32a) expresses it, " the 

 power possessed by living organisms alone of reacting to the 

 most various external stimuli in one way or another." It is 

 chiefly through this irritability that living objects can be distin- 

 guished from non-living ones, and in consequence the earlier 

 natural philosophers considered that it was the expression of a 

 special vital force which was only to be seen in organised nature. 



Modern science has discarded the theory of vitalism (vitalismus) ; 

 instead of explaining irritability by means of a special vital 

 force, it is considered to be a very complicated chemico-physical 

 phenomenon, differing only in degree from other chemico-physical 

 phenomena of inanimate nature. That is to say, the external 

 stimuli come into contact with a substance very complex in 

 structure, an organism, which is an exceedingly complicated 

 material system, and in consequence they give rise to a series of 

 very complex phenomena. 



However, care must be taken in accepting this mechanical 

 conception not to fall into the very common mistake of trying to 

 explain vital processes as being due directly to mechanical causes, 

 in consequence of their analogy to many phenomena seen in 



1 Claude Bernard (IV. la), in bis lectures on vital phenomena, arrives at the 

 same conclusion, his opinion being based on a number of considerations : 

 " Arrives au terme de nos etudes, nous voyons qu'elles nous imposent une 

 conclusion tres-generale, fruit de 1'experience, c'est, a savoir, qu'entre les 

 deux ecoles qui font des phenonemes vitaux quelque chose d'absolument 

 distinct des phenone'mes physico-chimiques ou quelque chose de tout a fait 

 identique a eux il y a place pour une troisietne doctrine, celle du vitalisuie 

 physique, qui tient compte de ce qu'il y a de special dans les manifestations de 

 la vie et de ce qu'il y ade conforme a 1'action des forces generates : Telemeiit 

 ultimo du pheuomene est physique ; 1'arrangement est vital ! " 



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