344 SECRETION. 



this principle to the glands generally, both secretory 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 constitu- 

 ents of the urine. 



With regard to the origin of the principles peculiar to the true secretions, it is impos- 

 sible to entertain any other view than that they are produced in the epithelial structures 

 of the glands ; and the old idea that they exist ready-formed in the blo'od cannot be 

 maintained. While the secretions contain inorganic salts in solution, transuded from the 

 blood, the organic constituents, such as pepsin, ptyaline, pancreatine, etc., are readily 

 distinguished from all other albuminoid principles by their peculiar physiological proper- 

 ties; although some of them are apparently identical with albumen in their ultimate 

 composition and in most of their chemical reactions. 



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

 the true secretions, as contradistinguished from the excretions, are formed de now by the 

 epithelial structures of the glands, out of material furnished by the blood. Their forma- 

 tion is by no means confined to what is usually termed the period of functional 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 that the formation of the elements of the secretions takes place 

 with fully 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 inter- 

 vals of repose and are capable of discharging their secretions for a limited time only. 



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

 membrane, or the pancreas, upon the introduction of food into the alimentary 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 elements of the 

 secretion. This difference in the vascularity of the glands during their activity is very 

 marked when the organs are exposed in a living animal and is one -of the important facts 

 bearing upon the mechanism of secretion. Beaumont observed this in his experiments 

 on St. Martin and was the first to show conclusively that the gastric juice is secreted only 

 when food is taken into the stomach or when some stimulus is applied to its mucous 

 membrane. Bernard, in his experiments upon the pancreas, noted the pale appearance 

 of the gland during the intervals of digestion and its reddened and congested condition 

 when the secretion flowed from the duct ; and these observations have been confirmed 

 by all who have experimented upon the glands in living animals. 



In later experiments upon the circulation in the salivary glands and its relation to 

 secretion, Bernard has fully investigated the variations in the vascular supply to the 

 glands, with the most definite and satisfactory results. His observations were made 

 chiefly upon the submaxillary gland in dogs ; and he has shown that, during the func- 

 tional activity of this organ, if a tube be introduced into the vein, the quantity of blood 

 which may be collected in a given time is four or five times that which is discharged in 

 the intervals of secretion. It was ascertained, also, that the venous blood coming from 

 the gland contained much less water than the arterial blood ; and, on comparing the 

 quantity of water lost by the blood in its passage through the gland in a given time with 

 the quantity discharged in the saliva, they were found to exactly correspond. 



The differences in the quality and the composition of the blood corning from the 

 glands during their repose and their activity have an important bearing upon the mech- 

 anism of secretion. As far as the composition is concerned, these differences appear to 

 be dependent mainly upon the modifications in the circulation. When the gland is in 

 repose, the blood coming from it has the usual dark, venous hue and contains the ordinary 

 proportion of carbonic acid ; but, during secretion, when the quantity of blood passing 

 through the organ is increased, the color is nearly as bright as that of arterial blood, and 

 the proportion of carbonic acid is very small. At this time, also, the blood is frequently 



