CHAPTER XV. 

 SECRETION. 



Secretion. As the blood flows through the capillaries of the 

 body certain of its nutritive principles are separated by the activity 

 of the epithelial cells of the capillary wall, aided by the physical 

 processes of nitration and diffusion. To this p/ocess the term 

 secretion may be applied. This separated or secreted material may 

 be utilized in several ways: 



1. For the repair of the tissues, for growth, for the liberation of 



energy. 



2. For the elaboration or production by specialized organs of a 



variety of complex fluids of widely different application. The 



fluids thus formed are utilized for the most part to meet some 



special need of the body. All such fluids are termed secretions. 



The organs concerned in the elaboration of the various secretions 



are covered or lined by epithelium to the activity of which the 



secretion is to be referred. To all these organs the term gland may 



be applied. As these fluids are poured out on the surface of the 



body, they have been termed external secretions:^, g., mucus, saliva, 



gastric-trace ? milk, setmteHJusliiatter, etc Within recent years it nas 



beerT demonstrated that trie epithelium of glands and particularly 



of those which do not possess a duct, such as the thyroid, jh^mus, 



adrena^^jiy^Kiph^gis, etc., also produces certain specific constituents 



y^hich^arereabsorbed into the blood, and which in some unknown^ 



A-but yet favorable way influence the general nutrition. To such prod- 



~lf ijc.t,fi of tbp.gp glands the term internal secretions has been given. 



The blood, in addition tb" its^nutritive ^constituents, contains a 

 number of principles, derived from tjie tissues, which are to be 

 regarded as waste products, the outcome of the metabolic activity 

 of the tissues and of no further use to the body. If retained, they 

 would seriously if not fatally interfere with the normal physiologic 

 activities of the different tissues. They are therefore removed by 

 specialized organs after their separation from the blood-stream. The 

 waste products in solution thus removed are not capable of being 

 utilized for any special purpose, and are therefore termed excrj^ 

 tions: e. g., urine^ersrjiration, etc. Excretion, however, is per- 

 lormeo! by tlie activities of epithelial cells aided by the physical 

 forces of diffusion and filtration; and though a distinction is made 



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