8 PROTOPLASMIC ACTION AND NERVOUS ACTION 



3. IRRITABILITY 



It is characteristic of all organisms that they respond 

 to changes in their environment (stimuli) by changes in 

 their own activity (response). And since metabolic 

 reactions underlie all vital activity, this fact implies 

 that the chemical reactions constituting metabolism 

 are subject to the influence of external agencies acting 

 upon the protoplasm. Both constructive and destruc- 

 tive metabolism may be thus influenced. 



In general, what we mean by irritability is this 

 susceptibility to external influence; irritability, however, 

 cannot be considered as a special property independent 

 of the continual automatic, chemical, and other activity 

 of the living system; its existence merely shows that 

 the chemical reactions of protoplasm are subject to 

 modification — e.g., acceleration or the reverse — under the 

 influence of relatively slight changes of state, caused usually 

 by the action of external agencies upon the protoplasm. 



A peculiarity of most intact organisms is that the 

 changes of activity thus induced are normally of such 

 a character as to favor the continued existence of the 

 individual or of the species in the environment; this 

 general fact may be expressed by saying that the normal 

 responses to stimulation, however varied in detail, have 

 a regulative or adaptive character. Adaptiveness, how- 

 ever, is a peculiarity of the organism as a whole, not an 

 inherent property of protoplasm in general; this is 

 shown by the fact that isolated parts may show irrita- 

 bility quite independently of any adaptive reference; 

 e.g., nerve or muscle. In this respect irritability may be 

 compared with the chemical instability of explosives, 

 which may also be applied adaptively. 



