128 The Unity of the Organism 



both as to the essential nature of each, considered individ- 

 ually, and as to their relation with one another, by a number 

 of exceedingly powerful and subtle chemical substances (in- 

 ternal secretions) which are produced by certain parts, are 

 passed into the blood, and by it are carried about over the 

 whole organism to exert their appropriate influences on 

 other parts and functions. Because of the peculiar way these 

 substances do their work, they have been called chemical 

 messengers, or to have a distinctive name, hormones. 



Relation Between the Internal Secretory and Nervous 

 Systems 



But no physiological truth is better known than that one 

 of the main offices of the nervous system is to correlate the 

 organs and parts of the body with one another. It is but 

 natural to suppose, therefore, that if there is a chemical 

 scheme for accomplishing the same end, the two are in some 

 way closely related. 



That the relations which exist between the cerebro-spinal 

 nervous system, the autonomic nervous system (including the 

 sympathetic), and the internal secretions, constitute one of 

 the most important subjects in the whole physiological do- 

 main, at the same time that it is one of the most recondite 

 and difficult to investigate, has come gradually to view 

 through the work of the last few decades. We will try to 

 extract enough from the mass that has been written on the 

 subject, to illustrate the principles involved. The modern 

 period of knowledge of what was formerly but rather in- 

 definitely included under the term sympathetic nervous sys- 

 tem, has revealed that we have to do with a portion of the 

 general nervous mechanism which in reality is a subdivision 

 of a larger category. 



