ASSIMILATION 57 



Enzymes in the Tissues. — These enzymes are not con- 

 fined to the digestive juices. If they were, the change 

 of food substances into protoplasm might be more com- 

 plicated than it is. They may exist in any part of the 

 body. Through their reversible action soluble substances 

 may be changed into proteids, fats or starch, and stored 

 in any part of the body ; or proteids, starch or fats may 

 be changed into soluble form and carried by the blood to 

 other parts of the body where they may be used by the 

 protoplasm. Large quantities of proteids are stored in 

 the muscles and in the blood ; starch in the muscles 

 and in the liver ; and fat in any part of the body, though 

 it is found in quantities most often just under the skin. 

 In cases of starvation, these stored substances may be 

 drawn upon, the fat first, and later the starches and 

 proteids. 



Passage of Food Through the Canal. — Through a 

 series of muscular contractions, food taken into the 

 mouth is forced to pass down the canal. The tongue and 

 cheek muscles help to hold the food between the teeth 

 while it is chewed ; they then squeeze it into the cavity 

 at the back of the mouth, over the top of the windpipe 

 which is closed by a swinging lid, into the oesophagus, 

 which relaxes to receive it. The oesophagus then con- 

 tracts behind the food and relaxes before it, until it 

 reaches the stomach, a small pear-shaped bag, which 

 holds about three pints when it is moderately distended. 



During the first hour and a half the opening into the 

 small intestine is very small and only the dissolved 

 starches and proteids pass through. Later the opening 

 becomes larger and other substances pass through easily. 



