PRODUCTION OF THE SECRETIONS. 309 



ters and transforms them into its own substance. The exact nature of this 

 property is unexplained. 



In all of the secreting organs, epithelium is found which seems to possess 

 the power of forming the peculiar constituents of the different secretions. 

 The epithelial cells lining the tubes or follicles of the glands constitute the 

 only peculiar structures of these parts, the rest being made up of basement- 

 membrane, connective tissue, blood-vessels, nerves, and other structures which 

 are distributed generally in the economy ; and these cells alone contain the 

 constituents of the secretions. It has been found, for example, that the 

 liver-cells contain the glycogen formed by the liver ; and it has been farther 

 shown that when the cellular structures of the pancreas have been destroyed, 

 the secretion is no longer produced. There can be hardly any doubt with 

 regard to the application of this principle to the glands generally, both secre- 

 tory and excretory. Indeed, it is well known to pathologists, that when the 

 tubes of the kidney have become denuded of their epithelium, they are no 

 longer capable of separating from the blood the peculiar constituents of the 

 urine. 



With regard to the origin of the characteristic constituents to the true 

 secretions, it is impossible to entertain any other view than that they are pro- 

 duced in the epithelial structures of the glands. While the secretions con- 

 tain inorganic salts in solution transuded from the blood, the organic con- 

 stituents, such as ptyaline, pepsine, trypsine etc., are readily distinguished 

 from all other albuminoid substances, by their peculiar physiological proper- 

 ties. 



It may be stated, then, as a general proposition, that the characteristic 

 constituents of the true secretions, as contradistinguished from the excre- 

 tions, are formed by the epithelial structures of the glands, out of materials 

 furnished mainly by the blood. Their formation is by no means confined to 

 what is usually termed the period of activity of the glands, or the time when 

 the secretions are poured out, but it takes place more or less constantly when 

 no fluid is discharged. It is more than probable, indeed, that the formation 

 of the peculiar and characteristic constituents of the secretions takes place 

 with as much activity in the intervals of secretion as during the discharge 

 of fluid ; and most of the glands connected with the digestive system seem 

 to require certain intervals of repose and are capable of discharging their 

 secretions for a limited time only. 



When a secreting organ is called into activity like the gastric mucous 

 membrane or the pancreas, upon the introduction of food into the aliment- 

 ary canal a marked change in its condition takes place. The circulation 

 in the part is then very much increased in activity, thus furnishing water 

 and the inorganic constituents of the secretion. This difference in the quan- 

 tity of blood in the glands during their activity is very marked when the' 

 organs are exposed in a living animal, and is one of the important facts bear- 

 ing upon the mechanism of secretion. 



In all the secretions proper, there are intervals, either of complete re- 

 pose, as is the case with the gastric juice or the pancreatic juice, or periods 



