196 How a Complex Animal Uses Food unit iv 



by peristalsis they are forced through intermediate products. If you wish to 

 the opening, a small amount at a time, know the details and the names of en- 

 After several hours the last of the meal zymes they are as follows: the enzymes 

 will have been delivered to the small in- which split fats are Upases — ly'paces. 

 testine. The stomach is empty and ready They change fats into fatty acids and 

 to receive more food when the next glycerin. The starch is converted by a 

 mealtime comes. It is important to know starch-splitting enzyme called amylase 

 that fats slow up secretion of juices and — am'i-lace — into a complex sugar which 



muscle contraction; a meal rich in fats, 

 therefore, will remain in the stomach for 

 a longer time. 



Food in the small intestine. Let us take 

 stock of what changes have occurred 

 when the food has reached the small in- 

 testine. A good many of the minerals 

 have been made soluble by hydrochloric 

 acid in the stomach. A very little of the 

 starch has been changed into a complex 

 sugar by saliva in the mouth. Most of 

 the sugars are just as they were when 

 eaten. Some of the proteins have been 

 split up into peptones by pepsin in the 

 stomach. Fats have, probably, scarcely 

 been touched. Much of the food has not 

 been acted on at all; some has been par- 

 tially changed but it is not yet completely 

 split up into compounds simple enough 

 to be used. 



Having taken stock of what happened 

 before food arrives in the small intestine, 

 let us trace the food further. In the first 

 part of the small intestine the food comes 

 in contact with pancreatic juice, intes- 

 tinal juice, and bile. Let us note the effect 

 of each of these in turn. The pancreatic 

 juice has three types of enzymes: one 

 kind acts on proteins, one on starch, and 

 one on fats. The fats are changed by fat- 

 splitting enzymes into end products. The 

 starch is converted into complex sugar, 

 an intermediate product. Proteins, even 

 peptones, are changed into still simpler 



is still an intermediate product, not the 

 kind of sugar a cell can use. Proteins and 

 peptones which had been formed in the 

 stomach are changed into intermediate 

 products even smaller than peptones, but 

 still intermediate products. (One of the 

 enzymes in the pancreatic juice that does 

 this is tryps'm — tv\^' sin.) 



You can see that pancreatic juice does 

 not complete the job of digestion. Much 

 of the food is still insoluble. The intes- 

 tinal juice which works in partnership 

 with the pancreatic juice completes di- 

 gestion. Sunk into the walls of the small 

 intestine are microscopic glands like gas- 

 tric glands; these secrete intestinal juice 

 which contains three kinds of enzymes. 

 One kind (erepsin) makes amino acids, 

 which are end products, out of the pro- 

 tein intermediate products. Another kind 

 breaks down complex sugars into the 

 end product, glucose or other simple 

 sugar. The third acts on the fats not yet 

 digested by the pancreatic juice. Thus, 

 while the food is in the small intestine it 

 can be completely broken down into end 

 products that can find their way throuq-h 

 the intestinal walls and into the blood. 



The bile from the liver contains no 

 enzymes but it aids in preparing fats for 

 digestion and it is important in various 

 other ways. 



Preparation of fats for digestion. When 

 fats are warmed by the heat of the body 



