DIVISION OF SECRETIONS. IT 



fluid, and may be eliminated, either constantly or occasion- 

 all v, by a number of organs. As they are produced con- 

 tinually in the substance of the tissues and taken up by 

 the blood, they are constantly discharged into the substance 

 of the proper eliminating organs. "When the glands which 

 thus eliminate these principles are destroyed, or their func- 

 tion seriously impaired, the excrementitious matters may 

 accumulate in the blood, and give rise to certain toxic 

 phenomena. These effects, however, are often retarded 

 by the vicarious discharge of such principles by other 

 organs. 



There are some fluids, as the bile, which perform impor- 

 tant functions as secretions, and which nevertheless contain 

 certain excrementitious matters. In these instances it is 

 only the excrementitious matters that are discharged from 

 the organism. 



In the serous sacs, the sheaths of tendons and of muscles, 

 the substance of muscles, and some other situations, are found 

 fluids which simply moisten the parts, and which contain 

 very little organic matter and but a small proportion of in- 

 organic salts. Although these are frequently spoken of as 

 secretions, they are produced generally by a simple mechan- 

 ical transudation of certain of the constituents of the blood 

 through the walls of the blood-vessels. 1 Still, it is difficult 

 to draw a line rigorously between transudation and some of 

 the phenomena of secretion ; particularly as late experiments 

 upon dialysis have shown that simple osmotic membranes are 

 capable of separating complex solutions, allowing certain con- 

 stituents to pass much more freely than others. 3 This fact ex- 

 plains why the transuded fluids do not contain all the soluble 

 principles of the blood in the proportions which exist in the 

 plasma. All the secreted fluids, both the true secretions and 

 the excretions, contain many of the inorganic salts of the 

 blood-plasma. 



1 See vol. ii., Absorption, p. 505. 8 Ibid., p. 477. 



