THE NERVOUS FUNCTIONS 27 



entering a telephone exchange are gathered together in great 

 cables and distributed to the switchboards in accordance with a 

 carefully elaborated system, so in the body nerve-fibers of like 

 function tend to run together in separate nerves or within the 

 brain in separate bundles called tracts. Notwithstanding the 

 complexity of organization of the nervous organs, the larger and 

 more important functional systems of nervous pathways have 

 been successfully analyzed, and the courses of nervous discharge 

 from the various receptors to the appropriate centers of adjust- 

 ment, and from these (after manifold correlations with other sys- 

 tems) to the organs of response, are fairly well known. The 

 acquisition of this knowledge has required several centuries of 

 painstaking anatomical and physiological study, and much 

 remains yet to be done. 



The external forms of the brain and other parts of the nervous 

 system are dependent mainly upon the arrangements of the 

 nerve-cells of which they are composed (for the characteristics of 

 these cells see Chapter III), and these arrangements, in turn, 

 are correlated with the functions to be performed. The func- 

 tional connections of the nerve-cells can be investigated best by 

 the microscopical study of the tissues combined with physiolog- 

 ical experimentation. From this it follows that the study of the 

 gross anatomy, the microscopical anatomy (histology), and the 

 physiology of the nervous system should go hand in hand so far 

 as this is practicable. 



A study of the comparative anatomy of the nervous system 

 shows that its form is always correlated with the behavior of the 

 animal possessing it. The simplest form of nervous system con- 

 sists of a diffuse network of nerve-cells and connecting fibers 

 distributed among the other tissues of the body. Such a ner- 

 vous system is found in some jelly-fishes and in parts of the 

 sympathetic nervous system of higher animals. Animals which 

 possess this diffuse type of nervous system can perform only 

 very simple acts, chiefly total movements of the whole body 

 or general movements of large parts of it, with relatively small 

 capacity for refined activities requiring the cooperation of 

 many different organs. But even the lowest animals which 

 possess nerves show a tendency for the nervous net to be con- 

 densed in some regions for the general control of the activities 



