308 SECRETION. 



body. These may be classed as excretions, the urine being the type of fluids 

 of this kind. The characteristic constituents of the excrementitious fluids 

 are formed in the tissues, as one of the results of the constant changes going 

 on in all organized, living structures. They always pre-exist in the circulating 

 fluid and may be eliminated, either constantly or occasionally, by a number 

 of organs. As they are produced continually in the substance of the tissues 

 and are taken up by the blood, they are constantly separated from the blood 

 by the proper eliminating organs. When the glands which thus eliminate 

 these substances are destroyed or when their action is 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 

 action of other organs. 



There are some fluids, as the bile, which have important uses as secre- 

 tions, and which nevertheless contain certain excrementitious matters. In 

 these instances, it is only the excrementitious matters that are discharged 

 from the organism. 



In the sheaths of some tendons and of muscles, in the substance of mus- 

 cles and in some other situations, fluids are found which simply moisten the 

 parts and which contain very little organic matter, with but a small propor- 

 tion of inorganic salts. Although these are frequently spoken of as secretions, 

 they are produced generally by a simple, mechanical transudation of certain 

 of the constituents of the blood through the walls of the vessels. Still, it is 

 difficult to draw a line rigorously between transudation and some of the phe- 

 nomena of secretion ; particularly as experiments upon dialysis have shown 

 that simple, osmotic membranes are capable of separating complex solutions, 

 allowing certain constituents to pass much more freely than others. This 

 fact explains why the transuded fluids do not contain all the soluble con- 

 stituents of the blood in the proportions in which they 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. 



Mechanism of the Production of the True Secretions. Although the 

 characteristic constituents of the true secretions are not to be found in the 

 blood or in any other of the animal fluids, they can generally be extracted 

 from the glands, particularly during their intervals of so-called repose. This 

 fact has been repeatedly demonstrated with regard to many of the digestive 

 fluids, as the saliva, the gastric juice and the pancreatic juice ; and artificial 

 fluids, possessing certain of the physiological properties of the natural secre- 

 tions, have been prepared by simply extracting the glandular tissue with water. 

 There can be no doubt, therefore, that during the periods when the secre- 

 tions are not discharged, the glands are taking from the blood matters which 

 are to be transformed into the characteristic constituents of the individual 

 secretions, and that this process is constant, bearing a close resemblance to 

 the general act of nutrition. There are certain anatomical elements in the 

 glands, which have the power of selecting the proper materials from the 

 blood and causing them to undergo peculiar transformations ; in the same 

 way that the muscular tissue takes from the nutritive fluid albuminoid mat- 



