924 THE EXTERNAL SECRETIONS 



sists of a mixture of phloroglucin and vanillin dissolved in absolute alcohol, and 

 the latter of dimethylamino-azo-benzene. 



The first amount of hydrochloric acid secreted usually gives a negative reaction 

 with these reagents, because it is bound by the albuminous bodies to form acid 

 albuminates. Furthermore, while pure gastric juice contains no lactic acid, this 

 acid is always present in the gastric contents composed of the pure juice and a 

 mixture of partly digested food. It arises in consequence of the fermentation of 

 carbohydrates which are attacked by the bacillus lactici ingested with the food, 

 and are converted into sugar and lactic acid. Under ordinary conditions, however, 

 the action of this bacillus is cut short by the hydrochloric acid, because even an 

 acidity of only 0.07 to 0.08 per cent. HCl absolutely prevents the formation of 

 lactic acid from dextrose. Consequently, lactic acid must be formed chiefly during 

 the early stages of gastric digestion or when there is a deficiency in hydrochloric 

 acid, as during carcinomatous affections of the stomach. But naturally, the acidity 

 of normal gastric juice is not due to lactic acid, as may be proved by taking this 

 acid up with ether and applying Uffelmann's test^^to the extract. 



Pepsin (Th. Schwann, 1836) is not present in the cells of the gastric glands as 

 such, but in its inactive form, known as stored pepsin or pepsinogen. ^ The latter, 

 therefore, may be regarded as the precursor or mother-substance of this ferment 

 which assumes its active condition only after its escape from the cells and in the 

 presence of hydrochloric acid. This is proved by the fact that the mucous mem- 

 brane of the dog or pig, which is alkaline or neutral in reaction, may be extracted 

 with water and mixed with hydrochloric or some other acid (0.3 percent.) to form 

 a powerful digestive medium. Contrariwise, gastric mucosa extracted under 

 glycerin may be kept for some time without any indication of self-digestion or 

 autolysis. This process, however, sets in immediately if an acid is added to this 

 extract. Furthermore, Langley^ has shown that pepsin is very sensitive to 

 alkalies, because when neutralized with sodium hydrate and again acidified, it loses 

 much of its former potency. An extract of the mucosa, however, may be made 

 slightly alkaline for a short time without losing its activity on acidification, while 

 an acid extract cannot be made alkaline without permanently destroying its power. 

 Another means of showing that the substance contained in the gastric cells is 

 different from actual pepsin, is furnished by the fact that carbonic anhydrid 

 gas destroys the action of the pepsinogen contained in a neutral aqueous extract 

 of frog's esophagus. Contrariwise, if this extract is first acidified and then 

 neutralized, the passage of carbonic anhydrid through it does not nullify its 

 power. 



Pepsin is a colloidal substance. As such it is not dialyzed through animal 

 membranes or parchment paper. Regarding its chemical nature we know very 

 little. Pekelharing3 and Nencki and Lieber* classify it as a protein or protein- 

 like body of the elementary composition: C, 51.26 per cent., H, 6.74 per cent., 

 N, 14.33 per cent., and S, 1.5 per cent. It shoiild be remembered, however, that 

 this ferment varies in its composition in different animals, because it presents 

 not only certain differences in the optimum concentration of the acid required to 

 activate it, but also in its resistance to heat. The question of what becomes of the 

 pepsin after it has unfolded its ferment-action, cannot be answered with certainty. 

 The probability is that the largest amount of it is destroyed in the intestine by the 

 other enzymes or by the bacteria, but a small portion of it may also be absorbed 

 and enter the blood and urine. 



Rennin. — The gastric juice of mammals, as well as aqueous infusions of the 

 gastric mucosa, possesses the property of curdling milk. This process is essentially 

 a coagulation during which a soluble protein contained in milk is converted into its 



^ Hammarsten, Zeitschr. fiir Physiol. Chemie, xcii, 1914, 121. 



2 Jour, of Physiol, vii, 1886, 371. 



3 Zeitschr. fiir physiol. Chemie, xxxv, 1902, 8 

 < Ibid., xxxii, 1901, 261. 



