CHEMISTRY OF DIGESTION AND NUTRITION. 235 



further than the curdling ; the digestion of the curd is carried on by the pep- 

 sin, and later, in the intestines, by the trypsin, with the formation of proteoses 

 and peptones as in the case of other proteids. 



Action of Gastric Juice on Carbohydrates and Fats. The gastric juice 

 itself has no direct action upon carbohydrates ; that is, it does not contain an 

 araylolytic enzyme. It is possible, nevertheless, that some digestion of carbo- 

 hydrates goes on in the stomach, for, as has been seen, the masticated food is 

 thoroughly mixed with saliva before it is swallowed. The portion that enters the 

 stomach in the beginning of digestion, when the acidity of the contents is small 

 (see p. 227), may continue to be acted upon by the ptyalin. This effect, however, 

 cannot be considered important, since the acidity of the contents of the stomach 

 must soon reach a point sufficient to suspend, and then to destroy, the ptyalin. 

 It should be added, however, that Lusk l has shown that cane-sugar can be 

 inverted to dextrose and levulose in the stomach. The importance of this 

 process of inversion, and the means by which it is accomplished, will be 

 described more in detail when speaking of the digestion of sugars in the small 

 intestine (p. 247). Upon the fats also gastric juice has no direct digestive 

 action. According to the best evidence at hand, neutral fats are not split in 

 the stomach, nor are they emulsified or absorbed. Without doubt, the heat 

 of the stomach is sufficient to liquefy most of the fats eaten, and the move- 

 ments of the stomach, together with the digestive action of its juice on the 

 proteids and albuminoids with which the fats are often mixed, bring about 

 such a mechanical mixture of the fats and oils with the other elements of the 

 chyme as facilitates the more rapid digestion of these substances in the intestine. 



Action of Gastric Juice on the Albuminoids. Gelatin is, from a 

 nutritive standpoint, the most important of the albuminoids. Its nutritive 

 value is stated briefly on page 215. It has been shown that this substance is 

 acted upon by pepsin in a way practically identical with that described for the 

 proteids. Intermediate products are formed similar to the albumoses, which 

 products have been named gelatoses 2 or glutoses; 3 these in turn may be con- 

 verted to gelatin peptones. It is stated that the action of pepsin is confined 

 almost, if not entirely, to changing gelatin to the gelatose stage. The pro- 

 teolytic enzyme of the pancreatic secretion, however carries the change to the 

 peptone stage much more readily. 



Why does the Stomach not Digest Itself? The gastric secretion will 

 readily digest a stomach taken from some other animal, or under certain con- 

 ditions it may digest the stomach in which it is secreted. If, for instance, an 

 animal is killed while in full digestion, the stomach may undergo self-diges- 

 tion, especially if the body is kept warm. This phenomenon has been observed 

 in human cadavers. It has been shown also that if a portion of the stomach 

 is deprived of its circulation by an embolism or a ligature, it may be attacked 

 by the secretion and a perforation of the stomach-wall may result. How, 



1 Voit: Zeitschrift fur Biologie, vol. xxviii., 1891, p. 269. 



2 Chittenden and Solley : Journal of Physiology, vol. xii., 1891, p. 23. 



3 Klug: Pfluger's Archivfilr die gesammte Physiologic, vol. 48, 1891, p. 100. 



