CHAP. II.] PHYSICAL AND CHEMICAL PROPERTIES OF GASTRIC JUICE. 79 



SECT. 5. PHYSICAL AND CHEMICAL CHARACTERS OF THE 

 GASTRIC JUICE. 



Pure gastric juice, such as can be obtained by the stimulation of 

 the mucous membrane of the stomach of an animal in which a gastric 

 fistula has been successfully established, is a thin, usually colourless, 

 though sometimes, as in the dog, yellowish liquid, possessed of a very 

 acid reaction, and of a faintly acid mawkish taste, and of a peculiar 

 though not easily defined odour. 



It has a specific gravity, which varies between 1001 and 1010, 

 the specific gravity varying in the same animal with varying condi- 

 tions of the secretion. 



When boiled, the gastric juice is not coagulable, but ceases to be 

 active. When cooled to C. the gastric juice of warm-blooded ani- 

 mals ceases to exert its peculiar digestive powers. 



The gastric juice of man contains less than one per cent, of solid 

 matters, of which about two -thirds are organic and one-third 

 mineral. 



The gastric juice may be kept for weeks and months without ex- 

 hibiting any signs of putridity, and retaining its proteolytic activity. 

 It possesses considerable antiseptic properties, as may be observed by 

 moistening slightly putrid meat with the juice. This property is 

 believed to be due to the free acid which it contains. 



Tho The essential physiological attribute of the gastric 



Tne essen- . . / o . . & _ 



tial constitu- juice is the power 01 breaking down and dissolving a 

 ents of the large part of the solid proteid aliments and converting 

 astric juice them into so-called albumoses and peptones. This power 

 marized. d e p en d s upon the co-existence in the juice of an enzyme 

 termed pepsin and an acid which has been shewn to be either free 

 hydrochloric acid or a more complex conjugated acid formed by the 

 union of hydrochloric acid with an organic body, which, however, if 

 it exists, is readily dissociated with the evolution of hydrochloric acid. 

 Neither pepsin nor hydrochloric acid are active alone, but a mixture 

 of the two bodies, in the presence of a proper quantity of water and 

 at a suitable temperature, acts essentially as the normal gastric juice. 

 Whilst the enzyme pepsin is absolutely indispensable, the acid may 

 be replaced by other acids and yet proper digestion will take place. 



Besides the proteolytic ferment pepsin, the gastric juice in man 

 and certain other animals contains a milk-curdling ferment, which we 

 may term the curdling ferment or 'rennin' (Foster, Lea), and to 

 which the name Chymosin has also been given by Deschamps 1 . 

 Neither pepsin nor the rennet ferment have yet been isolated as 

 pure chemical bodies, but our knowledge of their properties is derived 

 from a study of solutions which contain them in a state of greater 

 or less purity. 



1 Hammarsten, Lehrbuch d. pliysiolog. Chemie, Wiesbaden 1891, T. 153. 



