CHAPTER 5 



PHYSIOLOGY OF THE VERTEBRATE ANIMAL: IRRITA- 

 BILITY 



Metabolism in the vertebrate animal has been discus.sed in 

 terms of the chemical reactions in the protoplasm of cells. 

 In such a complex organism there are systems of organs, each 

 speciahzed and functioning in some particular way to make 

 metabohsm possible in all the cells of the body. Thus, in the 

 digestive system, food is disintegrated into its simple compounds 

 by means of secretions from various glands, and is absorbed into 

 the vascular system, by way of which it is transported to all parts 

 of the body. The respiratory system and the circulation of the 

 blood provide for oxygen intake and distribution. Again, the 

 blood, together with the organs of excretion, eliminates waste 

 products of metabolism. An organism cannot live if any one of 

 these systems ceases to function, and it becomes abnormal if the 

 activities of its organs are not correlated in the usual way. That 

 is, the manifestations of life by the animal depend upon the coor- 

 dination of its many systems. 



We have seen that in some cases the unification of functions is 

 brought about by endocrines secreted by certain glands and 

 cai-ried in the blood. This is to be thought of as a chemical coor- 

 dination. Another and a far more important integrating mechan- 

 ism is the nervous system. Both chemical and nervous coor- 

 dinations are dependent on irritabihty, which has been defined as 

 the capacity of protoplasm to respond by internal reaction to 

 stimuh or changes in the environment. All protoplasm is irritable. 

 One problem of the multicellular animal is that of keeping individ- 

 ual cells in touch with the environment of all the cells. This has 

 been solved by means of the sense organs, with their highly differ- 

 entiated capacities for sensitivity to changes in the environment; 

 and by the nervous system, which is speciahzed for conductivity. 

 The nervous system of a vertebrate may be divided into two 



