228 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 which 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, a free organic acid is secreted by the 

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

 Maly, the HC1 is the result of a reaction between the phosphates and the 

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



NaH 2 PO 4 + NaCl = Na 2 HPO 4 + HC1 ; or, 

 3CaCl 2 + 2Na 2 HPO 4 = Ca 3 (PO 4 ) 2 + 4NaCl + 2HC1. 



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

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

 with the aid of a nucleo-albumin of acid properties which 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 haemoglobin 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 

 which 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 

 pepsin is in a moist condition, in the total destruction of the enzyme. Pepsin 

 has never been isolated in sufficient purity for satisfactory analysis. It may be 

 extracted, however, from the gastric mucous membrane by a variety of methods 

 and in different degrees of purity and strength. The commercial preparations of 

 pepsin consist usually of some form of extract of the gastric mucous membrane 

 to which starch or sugar of milk has been added. Laboratory preparations are 

 usually made by mincing thoroughly the mucous membrane and then extract- 



