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 CHAPTER XII. 



SECRETION-. 



SECEETION 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 being discharged from the body 

 as excrement. In the former case, both the separated 

 materials and the processes for their separation are termed 

 secretions ; in the latter, they are named excretions. 



Most of the secretions consist of substances which, pro- 

 bably, 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 ex- 

 cretions, on the other hand, commonly or chiefly consist of 

 substances which, as urea, carbonic acid, and probably 

 uric acid, exist ready-formed in the blood, and are merely 

 abstracted therefrom. If from any cause, such as exten- 

 sive disease or extirpation of an excretory organ, the sepa- 

 ration 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 towards the exterior, on ac- 

 count 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 secre- 

 tion, and must not be thus regarded. 



These circumstances, and their final destination are, 



