CHAPTER VIII. 



SECRETION. 



IT is the function of gland cells to produce by the metabolism of their 

 protoplasm certain substances called secretions. These materials are of 

 two kinds ; viz. , those which are employed for the purpose of serving 

 some ulterior office in the economy, and those which are discharged from 

 the body as useless or injurious. In the former case, the separated 

 materials are termed true secretions; in the latter they are termed excre- 

 tions. 



The secretions as a rule consist of substances which do not pre-exist 

 in the same form in the blood, but require special cells and a process 

 of elaboration for their formation, e.g., the liver cells for the forma- 

 tion of bile, the mammary gland-cells for the formation of milk. The 

 excretions, on the other hand, commonly consist of substances which 

 exist ready-formed in the blood, and are merely abstracted therefrom. 

 If from any cause, such as extensive disease or extirpation of an excre- 

 tory organ, the separation of an excretion is prevented, and an accumu- 

 lation of it in the blood ensues, it frequently escapes through other 

 organs, and may be detected in various fluids of the body. But this is 

 never the case with secretions; at least with those that are most elabo- 

 rated; for after. the removal of the special organ by which each of them 

 is manufactured, the secretion is no longer formed. Cases sometimes 

 occur in which the secretion continues to be formed by the natural 

 organ, but not being able to escape toward the exterior, on account of 

 some obstruction, is re-absorbed into the blood, and afterward discharged 

 from it by exudation in other ways; but these are not instances of true 

 vicarious secretions, and must not be so regarded. 



The circumstances of their formation, and their final destination, are, 

 however, the only particulars in which secretions and excretions can be 

 distinguished; for, in general, the structure of the parts engaged in 

 eliminating excretions is as complex as that of the parts concerned in the 

 formation of secretions. And since the differences of the two processes 

 of separation, corresponding with those in the several purposes and des- 

 tinations of the fluids, are not yet ascertained, it will be sufficient to 

 speak in general terms of the process. 



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