CHAPTER XXV 

 INTERNAL SECRETION 



IN an early chapter of this book it was stated that there 

 are two ways in which one organ of the body may exert 

 an influence upon another. The more familiar and more 

 studied method has been through the nervous connections 

 which are maintained between all organs and the brain 

 and cord. The existence of such ties provides for the 

 possibility of reflex action. By this means any part of the 

 organism may modify the behavior of any other part, not 

 by affecting it directly, but by stimulating or inhibiting 

 the centers which preside over the second organ. Some 

 account of the work of the central nervous system is to 

 be given hereafter. Before this is undertaken it is well to 

 pay some attention to the other way in which co-ordination 

 is promoted; namely, by the transfer of chemical products 

 from place to place through the agency of the circulation. 



This is the subject usually covered by the term internal 

 secretion. The word hormone has been used elsewhere to 

 denote an active substance generated in one place, but 

 destined to take effect in another. The secretin produced 

 in the walls of the upper part of the small intestine and 

 carried thence to the pancreas and the other digestive 

 glands to excite them to pour out their juices is an example. 

 So also is the contribution made by the pancreas to the 

 blood, which proves to be so important to the utilization of 

 sugar. In this case it seems to be chiefly in the muscles 

 that the hormone is valuable. Other instances of similar 

 interaction can now be given, and there is reason to an- 

 ticipate that the list of internal secretions will soon be 

 made longer than it is at present. 



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