X 84 THE VITAL PROCESSES 



Tissue Enzymes. The important part played by enzymes in the 

 digestion of the food has suggested other uses for them in the body. 

 It has been recently shown that. many of the chemical changes in the 

 tissues are in all probability due to the presence of enzymes. An illus- 

 tration of what a tissue enzyme may do is seen in the changes which 

 fat undergoes. In order for the body to use up its reserve fat, it must 

 be transferred from the connective tissue cells, where it is stored, to the 

 cells of the active tissues where it is to be used. This requires that it 

 be reduced to the form of a solution and that it reenter the blood. In 

 other words, it must be redigested. For bringing about these changes 

 a substance identical in function with the steapsin of the pancreatic 

 juice has been shown to exist in several of the tissues. 



Although this subject is still under investigation, it may be stated 

 with certainty that there are present in the tissues, enzymes that change 

 dextrose to glycogen and vice versa, that break down and build up the 

 proteids, and that aid in the oxidations at the cells. The necessity 

 for such enzymes is quite apparent. 



Summary. The digested nutrients are taken up by the 

 capillaries and the lymph vessels and transferred by two 

 routes to the circulation. In passing from the alimentary 

 canal into the circulation the more important of the foods 

 undergo changes which adapt them to the needs of the 

 body. Since materials are absorbed more rapidly than 

 they are used, means are provided for storing them and 

 for supplying them to the cells as their needs require. 

 Capability of storage is an essential quality of energy- 

 yielding foods ; and substances, such as alcohol, which lack 

 this quality are not adapted to the needs of the body. For 

 causing the chemical changes that occur in the storage of 

 foods, as well as the oxidations at the cells, the presence 

 of active agents, or enzymes, is necessary. 



Exercises. i. In what respects does the absorption of food 

 materials from the alimentary canal differ from the absorption of a 

 simple liquid by a solid? 



