CHAPTER X 

 THE ENDOCRINE SYSTEM 



The internal secretion of an organ includes all substances 

 formed in the organ as a result of metabolic activity and removed 

 from the organ by the blood stream. In the case of glands pro- 

 vided with ducts there is an external secretion removed from the 

 organ through the ducts. Thus the liver takes from the blood 

 certain substances which, after undergoing various transforma- 

 tions, are secreted as bile through the bile duct into the intestine 

 and utilized in part in the digestion of fats. At the same time 

 the liver takes other substances from the blood and transforms 

 them into glycogen and urea, which return as sugar and urea to 

 the blood stream. Bile is the external secretion of the liver, 

 while sugar and urea are its internal secretions. Similarly, the 

 pancreas produces a substance that is necessary for the utiliza- 

 tion, by oxidation, of sugar, but this substance instead of being 

 poured through the pancreatic duct into the intestine along with 

 the pancreatic juice — its external secretion — is absorbed by the 

 blood as an internal secretion. 



Endocrines or hormones are internal secretions, characterized 

 by their capacity to stimulate metabolic activity in other organs 

 or to influence the metabolism of the body as a whole in a rather 

 specific manner. It should be clear, however, that while hor- 

 mones are internal secretions, all internal secretions are not 

 hormones. Urea is an internal secretion of the liver but is not 

 a hormone, since it really represents one of the end products of 

 nitrogen metabolism that is removed from the circulation by the 

 kidneys. 



Endocrines have been studied principally in vertebrate animals, 

 particularly amphibians, birds, and mammals. In these forms it 

 has been shown that normal metabolism of the body as a whole 

 depends upon contributions to the blood stream of specific sub- 

 stances, endocrines, from various organs. Since the effects are 

 produced through chemical means, the endocrine system repre- 

 sents a chemical method of coordinating and controlling metab- 



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