290 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



membrane. 1 The chemistry of the production of free HC1 also remains unde- 

 termined. No free acid occurs in the blood or the lymph, and it follows, there- 

 fore, that it is manufactured in the secreting cells. It is quite evident, too, 

 that the source of the acid is the neutral chlorides of the blood ; these are in 

 some way decomposed, the chlorine uniting with hydrogen to form HC1 which 

 is turned out upon the free surface of the stomach, while the base remains 

 behind and probably passes back into the blood. The latter part of the pro- 

 cess, the passage of the base into the blood-current, enables us to explain in part 

 the facts, noticed by a number of observers, that the alkalinity of the blood is 

 increased and the acidity of the urine is decreased after meals. Attempts to 

 express the reaction that takes place in the decomposition of the chlorides 

 are still too theoretical to merit more than a brief mention in a book of this 

 character. According to Heidenhain, the cells secrete a free organic acid, 

 which then acts upon and decomposes the chlorides. According to Maly, 

 the TTC1 is the result of a reaction between the phosphates and the chlorides 

 of the blood, as expressed in the two following equations: 



NaH 2 P0 4 + NaCl = Na 2 HP0 4 + HC1 ; or, 

 3CaCl 2 + 2Na 2 HP0 4 = Ca 3 (P0 4 ) 2 + 4NaCl + 2HC1. 



A recent theory by Liebermann supposes that the mass action of the C0 2 

 formed in the tissues of the gastric mucous membrane upon the chlorides, 

 with the aid of a nucleo-albumin of acid properties that can be isolated 

 from the gastric glands, may account for the production of the HC1. Although 

 it is customary to speak of the HC1 as existing in a free state in the gastric 

 juice, certain differences in reaction between this secretion and aqueous solu- 

 tions of the same acidity have led to the suggestion that the HC1, or a part of 

 it at least, is held in some sort of combination with the organic (proteid) con- 

 stituents of the secretion, so that its properties are modified in some minor 

 points just as the properties of hemoglobin are modified by the combination in 

 which it is held in the corpuscles. The differences usually described are that 

 in the gastric juice or in mixtures of HC1 and proteid the acid does not dialyze 

 nor distil off so readily as in simple aqueous solutions. The peptones and 

 proteoses formed during digestion seem to combine with the acid very readily 

 — so much so, in fact, that in certain cases specimens of gastric juice taken 

 from the stomach, although they give an acid reaction with litmus-paper, may 

 not give the special color reactions for free mineral acids. In such cases, how- 

 ever, the acid may still be able to fulfil its part in the digestion of proteids. 



Nature and Properties of Pepsin. — Pepsin is a typical proteolytic enzyme 

 that exhibits the striking peculiarity of acting only in acid media; hence 

 peptic digestion in the stomach is the result of the combined action of pepsin 

 and HC1. Pepsin is influenced in its action by temperature, as is the case with 

 the other enzymes ; low temperatures retard, and may even suspend, its activity, 

 while high temperatures increase it. The optimum temperature is stated to be 

 from 37° to 40° C, while exposure for some time to 80° C. results, when the 



1 Friinkel : I'jluger's Archivfur die gesammte Physiologic, 1891, Bd. 48, S. 63. 



