CHAPTER X. 



SECRETION. 



Secretion is the process by which materials are separated from the 

 blood by the cells of secreting glands and membranes, and are either 

 elaborated for the purpose of serving some ulterior office in the economy, 

 or are discharged 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 cells and a 

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

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

 The excretions, on the other hand, commonly or chiefly consist of sub- 

 stances 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 organ by which each of 

 them is elaborated, the secretion is no longer formed. Cases sometimes 

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

 gan, but not being able to escape towards the exterior, on account of 

 some obstruction, is re-absorbed into the blood, and afterwards 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- 



