218 GENERAL ZOOLOGY 



A hormone has been prepared from the testis which stimulates 

 the growth of the accessory reproductive organs of the male. 

 As in the case of the ovarian hormones, its secretion is under the 

 control of the anterior pituitary. 



General Significance of the Endocrine System. — The secretions 

 of the endocrine organs furnish a chemical mechanism for the 

 functional coordination of various parts of the body. The other 

 general mechanism for this purpose is the nervous system with its 

 nervous pathways, along which impulses are conducted to and 

 from all parts of the body. Of the two, the chemical method is 

 perhaps older phylogenetically, but practically it is difficult in the 

 final analysis to draw a sharp line between them because the 

 functional activity of the nervous system rests upon a chemical 

 basis too. In the nervous system, however, the paths along 

 which nervous impulses travel are separated and insulated from 

 each other, while in the endocrine system the means of transport- 

 ing the exciting agents is the blood, which is a common vehicle for 

 all of them. In the nervous system some of the pathways have 

 become specialized to perform a single function, as for example 

 the optic nerve; in the endocrine system, specific substances are 

 all mingled in the blood stream, where they may augment, 

 counteract, or stimulate each other before reaching their func- 

 tional destinations. Both systems, however, are closely related 

 functionally and each loses its individuality in the correlation 

 of the body as a whole. 



