CHAPTER II 



DEFINITION AND LIMITATION OF THE TERM "INTERNAL 

 SECRETION " 



IT is obvious at the outset that the term " internal secretion " 

 ought to be employed in such a way that it corresponds as 

 far as possible to the term " external secretion," or secretion 

 by means of ducts. Secretion, as we have seen, is the prepara- 

 tion and setting free of certain substances, the raw material 

 for which is supplied by the circulating blood. Such secretion 

 is the function of certain specially differentiated cells the 

 secretory cells. 



It has already been mentioned that several authors, relying 

 upon the fact that the different organs and tissues of the body 

 have different functions, and therefore pour out into the blood 

 different metabolic products, have insisted that all these have 

 an internal secretion, and that this secretion is in each case a 

 specific one. But this, as pointed out by Kohn, is simply a 

 misuse of the term " secretion." Just as we have certain 

 tissues namely muscle and nerve highly specialized and 

 set apart for the functions of motility and conduction of 

 irritability, so we have certain other tissues also highly differ- 

 entiated and set apart for the purposes of secretion, and it is 

 only to these that we can with propriety ascribe the function. 

 Such are secretory cells and their accumulations, called 

 " glands." The secretory cells are in their origin and in their 

 character epithelial Secretory cells are highly specialized 

 epithelial cells. It is not necessary to insist on this criterion 

 in the case of externally secreting glands, because here it is 

 generally recognized ; but it is just as important in regard to 

 internal secretion if the term is to be defined with anything 

 approaching accuracy. The morphological sign of special 

 differentiation in gland cells is the presence of granules which 

 undergo periodical changes in number and position, according 



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