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CHAPTER XXXII. 



ON SECRETION. SECRETIONS AND EXCRETIONS. SECRETIONS WHICH 



AR.E EXCREMENTITIOUS. SECRETIONS WHICH ARE NOT EXCRE- 



MENTITIOUS, WHICH SERVE ULTERIOR PURPOSES. VICARIOUS SE- 

 CRETION. INGESTA AND EGESTA. ANATOMY OF SECRETING OR- 

 GANS GENERALLY. OF THE GLAND CELL. OF THE DUCTS OF 



GLANDS. 



THE function of secretion is that by which organised bodies 

 separate from some portion of their internal or external surfaces a 

 material which -thereby becomes free or loose, and capable of re- 

 moval, and which is likewise commonly known as the secretion 

 from such surfaces. It is not sufficient that this material should be 

 separated simply from the blood; for, if this were so, all the tissues 

 would be secretions ; but it must also be separated from a tissue of 

 a special kind, hence known as a secreting tissue. The blood, then, 

 furnishes the materials which are to form the secretions ; but cer- 

 tain tissues are required, without which, these materials could not 

 become secreted products; and the secreting tissues are anatomi- 

 cally arranged so as to have one surface adapted to set free, in the 

 form of secretions, the materials derived on the opposite surface 

 from the blood. 



A secretion may be solid or fluid, living or dead, near to, or dis- 

 tant from, the constituents of the tissues in chemical constitution, 

 capable of re-entering the blood, and being farther serviceable in 

 the oeconomy, or, requiring to be expelled from the system as useless 

 or hurtful. Thus, a great variety of very dissimilar products are 

 conveniently classed together as secretions, in virtue of their being 

 separated from the body by tissues provided for that purpose. 



Excretions. Some secretions consist mainly of substances re- 

 sulting from the waste of the tissues coincident with their nutrition 

 or renewal, and which are thrown off from the blood through the 

 secreting surfaces almost as soon as the capillaries receive them 

 from the tissues. Such are the urine, and portions of those matters 



