16 SECRETION. 



gations within the past few years have shown that these 

 differences are very distinct. 



Certain of the fluids are formed by special organs, and 

 have important functions to perform, which do not involve 

 their discharge from the organism. These may be classed 

 as the true secretions ; and the most striking examples of 

 them are the digestive fluids. Each one of these fluids is 

 formed by a special gland or set of glands, which generally 

 has no other function ; and they are never produced by any 

 other part. It is the gland which produces the characteris- 

 tic element or elements of the true secretions out of materials 

 furnished by the blood ; and the principles thus formed never 

 preexist in the circulating fluid. The function which these 

 fluids have to perform is generally intermittent ; and when 

 this is the case, the flow of the secretion is intermittent, tak- 

 ing place only when its action is required. "When the parts 

 which produce one of the true secretions are destroyed, as 

 may be sometimes done in experiments upon living animals, 

 the characteristic elements of this particular secretion never 

 accumulate in the blood, nor are they formed vicariously 

 by other organs. The simple effect of such an experiment 

 is absence of the secretion, and the disturbances consequent 

 upon the loss of its function. 



Certain other of the fluids are composed of water, holding 

 one or more characteristic principles in solution, which re- 

 sult from the physiological waste of the tissues. These prin- 

 ciples have no function to perform in the animal economy, 

 and are simply separated from the blood to be discharged 

 from the body. These may be classed as excretions ; the 

 urine being the type of fluids of this kind. The characteristic 

 principles of the excrementitious fluids are formed in the tis- 

 sues, as one of the results of the constant nutritive changes 

 going on in all organized living structures. They are not pro- 

 duced in the glands by which they are eliminated, but ap- 

 pear in the secretion as the result of a sort of elective filtra- 

 tion from the blood. They always preexist in the circulating 



