CHAPTER XVIII* 

 SECRETION AND EXCRETION 



IN the circulation of the blood, not only are the 

 nutrient materials taken around through the body to be 

 used in the construction of various tissues, but certain 

 special fluids are taken up and conveyed to the external 

 or internal surfaces in the body, where, in glandular 

 structures, further elaboration takes place. The result- 

 ing products are of two kinds : some, like saliva, gastric 

 juice, bile, milk, etc., are for useful purposes ; others, 

 like sweat and urine, are expelled from the system as 

 useless or injurious. The separation of the former is 

 called secretion ; the removal of the latter is excretion. 

 Both processes are substantially alike. 



In the lower forms, there are no special organs, but 

 secretion and excretion take place from the general 

 surface. The simplest form of a secreting organ closely 

 resembles that of a respiratory organ, a thin membrane 

 separating the blood from the cavity into which the 

 secretion is to be poured. Usually, however, the cells 

 of the membrane manufacture the secretion from ma- 

 terials furnished by the blood. Even in the higher 

 animals, there are such secreting membranes. The 

 membranes lining the nose and alimentary canal and 

 inclosing the lungs, heart, and joints, secrete lubricating 

 fluids. 



The infolding of such a membrane into little sacs or 

 short tubes (follicles), each having its own outlet, is the 



* See Appendix. 

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