DIGESTION IN THE SMALL INTESTINE 95 



contains too little, the muscles are not stimulated to activity 

 and digestion is slow. If it is too full, the contractions cannot 

 take place, the food is insufficiently mixed with the ferments 

 and remains undigested, giving a feeling of sickness. 



Digestion in the small intestine. Saliva changes starch 

 to sugar, and pepsin and rennin change protein to peptones, 

 but only a small portion of the starch and protein food we eat 

 is digested in the mouth and stomach. This is because food 

 remains in the mouth only a few seconds and lies in the stomach 

 only a few hours, and is acted upon by saliva and gastric juice 

 for too short a time to undergo thorough transformation 

 into new substances. Fats are not changed at all by either 

 pepsin or ptyalin and require for digestion ferments not found 

 in either mouth or stomach. Undigested fats' and partially 

 digested starch and proteins pass from the stomach into 

 the small intestine where digestion is continued and com- 

 pleted by means of intestinal ferments. The small intestine 

 is a narrow, much-coiled, muscular tube about twenty feet 

 long, in which three digestive juices are abundant: the bile 

 sent to it from the liver, the pancreatic juice sent to it from the 

 pancreas, and the intestinal juices secreted by glands in its own 

 walls. Each of these juices is essential to intestinal digestion, 

 but the pancreatic juice plays the most important part and 

 carries on the main work of digestion. It changes starch to 

 sugar, converts proteins into peptones, and splits up fats into 

 substances fit for absorption and bodily use. The intestinal 

 juice also contains ferments which assist in the transformation 

 of starch to sugar, protein to peptones, and fats to soluble sub- 

 stances, but it is less important than the pancreatic juice. 

 Bile, unlike intestinal and pancreatic juices, has no direct 

 digestive power, but it is essential to healthy digestion. It 

 stimulates the walls of the intestines to muscular action and 

 assists in the movement of the food ; it also acts as a stimulus 



