MECHANISM OF SECRETION. 19 



gard to some of the glands, this has been satisfactorily de- 

 monstrated. It has been found, for example, that the liver- 

 cells contain the glycogenic matter formed by the liver ; 1 and 

 it has been further shown that when the cellular structures 

 of the pancreas have been destroyed, the secretion is no 

 longer produced. 2 There can be hardly any doubt with re- 

 gard to the application of this principle to the glands gener- 

 ally, both secretory and excretory. Indeed, it is well known 

 to pathologists, that when the tubes of the kidney have be- 

 come 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 principles peculiar to 

 the true secretions, it is impossible 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 blood, though adopted by some physiologists of the 

 present day, 8 cannot be maintained. While the secretions 

 contain inorganic salts transuded in solution from the blood, 

 the organic constituents, such as pepsin, ptyaline, pancrea- 

 tine, etc., are readily distinguished from all other albuminoid 

 principles by their peculiar physiological properties ; al- 

 though 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 contradistin- 

 guished from the excretions, are formed de novo by the epi- 

 thelial structures of the glands, out of material furnished by 

 the blood ; and that their formation 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, 



1 SCHIFF, De la nature des granulations qui remplissent les cellules hepatiques : 

 Amidon animale. Comptes rendus, Paris, 1859, tome xlviii., p. 880. 



2 BERNARD, Memoire sur le pancreas, Paris, 1856, pp. 17 and 69. 



3 MILNE-EDWARDS, Lecons sur la physiologie^ Paris, 1862, tome vii., p. 282. 



