270 SECRETION 



It is difficult to draw a line rigorously between transudation and 

 some of the phenomena of secretion ; particularly as experiments on 

 dialysis have shown that simple osmotic membranes are capable of sepa- 

 rating complex solutions, allowing certain constituents only to pass. 

 This fact explains why the transuded liquids do not contain all the solu- 

 ble constituents of the blood in the proportions in which they exist 

 in the plasma. All secreted liquids, both the true secretions and the 

 excretions, contain many of the inorganic salts of the blood-plasma. 



The secretions proper usually are classified as serous (albuminous) 

 and mucous ; the former being entirely liquid and the latter presenting 

 more or less viscidity. It is thought, also, that cells producing these 

 two kinds of secretions differ somewhat from each other in their histo- 

 logical characters. 



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 liquids, they often can be 

 extracted from the glands, particularly during their intervals of so-called 

 repose. This has been repeatedly demonstrated in regard to many of 

 the digestive secretions, as the saliva, the gastric juice and the pan- 

 creatic juice ; and artificial liquids, possessing certain of the physiologi- 

 cal properties of the natural secretions, have been prepared by simply 

 extracting the glandular tissue with an appropriate menstruum. There 

 can be no doubt, therefore, that during the periods when the secretions 

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

 are to be transformed into the characteristic constituents of the individ- 

 ual secretions, and that this is constant, bearing a close resemblance 

 to the general process of nutrition. Certain anatomical elements in 

 the glands have the power of selecting proper materials from the blood 

 and causing them to undergo peculiar transformations ; in the same 

 way that the muscular tissue takes from the blood albuminous matters 

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

 process is not understood. 



In all secreting organs, epithelium is found which produces the pe- 

 culiar 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, connec- 

 tive tissue, bloodvessels, nerves, and other structures that 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 further shown 

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

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



