128 THE VERTEBRATE ANIMAL: IRRITABILITY 



results from an alteration of cell metabolism which affects the 

 junction between the nerve and the effector. It therefore appears 

 that, like receptors, effectors are highly specialized and react in a 

 particular way or not at all. 



Coordination and Irritability 



It must not be forgotten that coordination in any organism 

 depends in the last analysis upon the irritability of protoplasm. 

 The unicellular animals respond directly to changes in their envi- 

 ronment which are received and become effective in the protoplasm 

 of the same cell. That certain of the individual cells of multi- 

 cellular organisms retain the capacity to respond directly to changes 

 in their immediate surroundings is clear from the nature of chemical 

 coordination. It will be recalled that endocrines carried in the 

 blood and lymph produce reactions by direct stimulation. How- 

 ever, it is impossible for each cell in a highly organized multicellular 

 animal to be directly stimulated by changes in the environment of 

 the organism as a whole. Integration of the many-celled animal 

 is accomplished by means of the mechanism of the neurone arcs of 

 the nervous system, connecting receptors and effectors according 

 to the principle of the common path. This coordination of higher 

 forms is associated with great specialization of cells and regions. 

 A particular receptor receives only a certain kind of stimulus; 

 neurones conduct impulses in only one direction; and effectors 

 respond in a distinctive manner to discharged impulses. The 

 question of the fundamental nature of this specialization is one that 

 cannot be answered at present. Nor, for that matter, can the 

 essential character of irritability be clearly explained. Certain 

 facts point to a close association between metabolism and irrita- 

 bility. In other words, if the metabolism of a cell is altered its 

 irritability may be changed. Metabolism, in turn, depends in 

 some way upon the physico-chemical nature of protoplasm. The 

 complete explanation of irritability and coordination, therefore, 

 awaits further knowledge of the physics and chemistry of proto- 

 plasm. It is, fortunately, quite possible to understand to a greater 

 or lesser degree the outward expression of protoplasmic irritability 

 in the behavior of animals. 



