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4 The Endocrine Organs 



not to be produced by a special structure, certainly not by a special organ, 

 but, so far as one can tell, by the ordinary cells which line the mucosa and 

 extend into its glands. 



A similar but not identical internal secretion was shown by Edkins to 

 be produced by the cells of the mucous membrane of the pyloric end of 

 the stomach. When rendered active and absorbed into the blood, this 

 secretion, which is termed gastrine, stimulates not the cells of the pancreas, 

 but those of the fundic glands of the stomach itself. 



A yet more remarkable example of the coincidence of external and in- 

 ternal secretory functions in the same organ is supplied by the generative 

 glands (ovary and testicle). It has been known from time immemorial — 

 the experiment is repeated daily for commercial purposes in thousands of 

 animals, and is still practised upon man for domestic reasons in certain 

 Oriental countries — that the removal of the generative glands in the young 

 male animal usually entirely prevents the development of the accessory 

 generative organs, such as the prostate, and of the features which 

 characterise the male sex externally. These changes may be prevented by 

 successfully grafting a testicle in the castrated animal. 



Again, removal of the ovaries in the young female exerts a profound 

 influence over the organism and prevents the development of many female 

 characteristics. In some cases, even in the adult, removal or atrophy of 

 the ovaries has been noticed to lead to the development of male characters. 

 Instances of this have been often recorded in birds. In young mammals 

 removal of the ovaries is followed by arrest of development of the uterus. 

 This may be prevented by successfully transplanting the ovaries, or by 

 grafting an ovary from another animal of the same species into the 

 peritoneum or elsewhere. It seems clear, therefore, that the result of 

 removal is due in both male and female to the absence of the internal 

 secretion of the generative gland. 



The Nature of the Active Principles contained in the 

 Internal Secretions 



We have compared the active materials of the internal secretions, which 

 are formed within cells and passed out into the blood, to the active agents 

 of the ordinary secretions, which are directed on the exterior by means of 

 a duct. But the comparison cannot be pressed. In the case of the external 

 secretions, the active agents when present are always of the nature of a 

 ferment. They belong to the class of bodies which are known as enzymes. 

 The conditions and modes of action of these bodies are for the most part 

 familiar to the biologist. They occur not only in secretions, but in the 

 bioplasm of most if not of all cells ; indeed the chemical activity of the cell 

 in most cases depends upon its contained enzymes. Although far simpler 

 in chemical nature than the bioplasm by which they are produced, and in 

 no sense endowed with life, enzymes are very probably of a protein nature, 



