CHAPTER XI. 



SECRETION. 



Secretion is the process by which materials are separated from the 

 blood, and from the organs in which they are formed, for the purpose 

 either of serving some ulterior office in the economy, or of being dis- 

 charged from the body as useless or injurious. In the former case, the 

 separated materials are termed secretions; in the latter, they are termed 

 excretions. 



Most of the secretions consist of substances which, probably, do not 

 pre-exist in the same form in the blood, but require special organs and a 

 process of elaboration for their formation, e.g., the liver for the formation 

 of bile, the mammary gland for the formation of milk. The excretions, 

 on the other hand, commonly or chiefly 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 excretory organ, 

 the separation of an excretion is prevented, and an accumulation 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 elaborated; for after the 

 removal of the special organs by which any of them is elaborated, it 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-absojbed into the blood, 

 and afterward discharged from it by exudation in other ways; but these 

 are not instances of true vicarious secretion, and must not be thus 

 regarded. 



These circumstances, 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 excre- 

 tions 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 destinations of the 

 fluids, are not yet ascertained, it will be sufficient to speak in general 

 terms of the process of separation or secretion. 



Every secreting apparatus possesses, as essential parts of its structure, 

 a simple and almost textureless membrane, named the primary or base- 



