CHEMISTRY OF DIGESTION AND NUTRITION. 225 



usually believed that the main value of the saliva, to the human being and to 

 the carnivora at least, is that it facilitates the swallowing of food. It is impos- 

 sible to swallow perfectly dry food. The saliva, by moistening the food, not 

 only enables the swallowing act to take place, but its viscous consistency must 

 aid also in the easy passage of the food along the oesophagus. Among the 

 herbivora it is probable that the longer retention of food in the mouth gives 

 the saliva opportunity for more complete digestive action. 



0. GASTRIC DIGESTION. 



After the food reaches the stomach it is exposed to the action of the secre- 

 tion of the gastric mucous membrane, known usually as the gastric juice. The 

 physiological mechanisms involved in the production and regulation of this 

 secretion, and the important part played in gastric digestion by the movements 

 of the stomach, will be found described in other sections (Secretion, Move- 

 ments of Alimentary Canal). It is sufficient here to say that the secretion 

 of gastric juice begins with the entrance of food into the stomach. By means 

 of the muscles of the stomach the contained food is kept in motion for several 

 hours and is thoroughly mixed with the gastric secretion, which during this 

 time is exerting its digestive action upon certain of the food-stuffs. From time 

 to time portions of the liquefied contents, known as chyme, are forced into the 

 duodenum, and their digestion is completed in the small intestine. Gastric 

 digestion and intestinal digestion go more or less hand in hand, and usually 

 it is impossible to tell in any given case just how much of the food will 

 undergo digestion in the stomach and how much will be left to the action of 

 the intestinal secretions. It is possible, however, to collect the gastric secre- 

 tion or to make an artificial juice and to test its action upon food-stuffs by 

 digestions in vitro. Much of our fundamental knowledge of the digestive 

 action of the gastric juice has been obtained in this way, although this has 

 been supplemented, of course, by numerous experiments upon lower animals 

 and human beings. 



Methods of Obtaining Normal Gastric Juice. The older methods used 

 for obtaining normal gastric juice were very unsatisfactory. For instance, an 

 animal was made to swallow a clean sponge to which a string was attached so 

 that the sponge could afterward be removed and its contents be squeezed out ; 

 or there was given the animal to eat some indigestible material, to start the 

 secretion of juice by mechanical stimulation, the animal being killed at the 

 proper time and the contents of its stomach being collected. A better method 

 of obtaining normal juice was suggested by the famous observations of Beau- 

 mont 1 upon Alexis St. Martin. St. Martin, by the premature discharge of 

 his gun, was wounded in the abdomen and stomach.' On healing, a fistulous 

 opening remained in the abdominal wall, leading into the stomach, so that the 

 contents of the latter could be inspected. Beaumont made numerous interest- 

 ing and most valuable observations upon his patient, Since that time it has 

 become customary to make fistulous openings into the stomachs of dogs when- 



1 The Physiology of Digestion, 1833. 

 15 



