CHEMISTRY OF DIGESTION AND NUTRITION. 297 



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

 intestine (p. 308). 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, [ts nutritive 

 value is stated briefly on page 277. 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 or glutoses; 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, 

 then, under normal conditions, is the stomach protected from corrosion by its 

 own secretion? The question has given rise to much discussion, and in reality 

 it deals with one of the fundamental properties of living matter, for the 

 same question must be extended to take in the non-digestion of the small 

 intestine by the alkaline pancreatic secretion, the non-digestion of the digestive 

 tracts of the invertebrates, and the case of the unicellular animals in which 

 there is formed within the animal's protoplasm a digestive secretion that 

 digests foreign material, but does not affect the living substance of the cell. 

 In the particular case under consideration — namely, the protection of the 

 mammalian stomach from its own secretion — explanations of the following 

 character have been offered: It was suggested (Hunter) that the " principle 

 of life" in living things protected them from digestion. This suggestion 

 cannot be considered seriously at the present day, since it implies thai living 

 matter is the seat of a special force, the so-called " vital principle," different 

 from the forms of energy acting upon matter in general. Appeals of this 

 kind to an unknown force in explanation of the properties of living matter 

 are not now permissible in the science of physiology. Moreover, it was 

 shown by Bernard that the hind leg of a living frog introduced into a 

 dog's stomach through a fistula undergoes digestion. The same thing will 



