8 INTERNAL SECRETION 



duet* may act as agents of augmentation, or possibly inhibition, 

 in regard to the special activities of various organs and tissues. 

 One of the best examples of this is the excitation of the respira- 

 tory centre produced during asphyxia by the circulation in the 

 blood of decomposition products normally eliminated in the 

 expired air. Again, when a large amount of protein is absorbed 

 and digested, the products formed probably stimulate an in- 

 creased metabolism in the body generally. When this occurs, 

 urea is formed by the liver in larger amount, and stimulates the 

 kidneys to increased activity. 



In a certain sense, and in the direction just indicated, it is 

 evident that all the tissues and organs of the body form internal 

 secretions, for they all pass into the blood materials which have 

 been formed as products of their metabolism. Everything 

 which an organ or tissue absorbs from the blood and lymph it 

 gives out to them again in some form or other, except in so far 

 as it forms or separates a secretion which passes away by special 

 ducts. It is obvious that in this, the broadest sense of the 

 expression "internal secretion," nothing further is implied 

 than that the blood which leaves by the veins coming from an 

 organ or tissue contains different chemical substances from that 

 which enters by the arteries. A distinction might possibly be 

 made in some cases between katabolic products such as are 

 formed by all tissues, and synthetic products which are only 

 formed by some, and the term " internal secretion " might be 

 reserved for the latter. But this distinction could not be 

 maintained systematically, for it is quite conceivable that a 

 definite specific and powerful internal secretion might be 

 formed by a katabolic process. Some authors have included 

 intracellular enzymes among the internal secretions. These 

 are still more " internal " than the secretions usually so-called, 

 for they are not passed out of the cell in which they are pro- 

 duced. They differ from the exo-enzymes such as are found 

 in the secretions by reason of the fact that they are bound up 

 in the protoplasm of the cells, and, so long as the cells are alive, 

 can only exert their action intracellularly. When the cells die 

 tin- protoplasm breaks up, and the enzymes may pass into 

 solution. It is supposed that these enzymes are elaborate < 1 by 

 and used during the life of the protoplasm. It is possible that 

 in starvation they may bring about the solution of the tissue 

 protein-, and that the autolytic processes which take place 



