SOME OF THE LIFE PROCESSES 119 



canal is thin, but it is made of small bodies, or cells, which 

 are packed closely together. Water will pass through this 

 membrane easily, but certain other substances, although 

 in a liquid state, will not pass through so readily. Those 

 solutions which pass through an animal membrane readily 

 are called crystalloids; for example, solutions of salt or 

 sugar. Liquids which do not pass readily through an ani- 

 mal membrane are known as colloids ; for example, solu- 

 tions of meat juice or starch. The action of the digestive 

 fluid is a double one : it changes the state of the solid food 

 physically, by rendering it liquid ; it changes the state of 

 organic foods (both solid and liquid) chemically, giving at 

 the same time to each of the altered food substances a new 

 physical property, namely, that of being able to pass through 

 the intestine wall. 



For illustration, we may suppose that a piece of potato 

 which contains a great deal of starch is taken as food. In 

 the mouth and intestine the starch is changed physically by 

 being made liquid or partially so, and it is changed chemi- 

 cally into a sugar compound by the action of the enzyme 

 ptyalin, which is present in the saliva. In the latter state, 

 what was once starch passes readily through the wall of the 

 intestine. A protein substance like meat juice cannot pass, 

 except in a slight degree, through the intestine until it has 

 been changed chemically. This is done in the intestine by 

 the enzyme trypsin, which is produced from the secretions 

 of the pancreas. The changed substance is called a peptone. 

 The enzyme steapsin, also produced by the pancreas, sepa- 

 rates the fats into compounds known as glycerin and fatty 

 acid. The fatty acids combine chemically with the alkali 

 in the digestive tract, resulting in compounds similar to 

 soap ; the process is therefore called saponification. When 

 the proteins are changed to peptones, the starch to sugar, 

 and the fats to glycerin and soaps, then these organic foods 



