!8o THE VITAL PROCESSES 



is only gradually absorbed as this process takes place. 

 It may properly, on this account, be regarded as stored 

 material. That such storage is of advantage is shown by 

 the observed fact that substances which digest quickly 

 (sugar, dextrin, "predigested foods," etc.) do not supply 

 the needs of the body so well as do substances which, like 

 starch and proteids, digest slowly. Even substances digest- 

 ing quite slowly (greasy foods and pastry), since they can 

 be stored longer in the food canal, may be of real ad- 

 vantage where, from hard work or exposure, the body 

 requires a large supply of energy for some time. These 

 " stay by " the laborer, giving him strength after the more 

 easily digested foods have been used up. Storage by the 

 food canal is limited chiefly to the stomach. 



Regulation of the Food Supply to the Cells. The 

 storage of food materials is made to serve a second pur- 

 pose in the plan of the body which is even more important 

 than that of supplying nourishment to the cells during the 

 intervals when no food is being taken. It is largely the 

 means whereby the rate of supply of materials to the cells 

 is regulated. The cells obtain their materials from the 

 lymph, and the lymph is supplied from the blood. Should 

 food substances, such as sugar, increase in the blood beyond 

 a low per cent, they are converted into a form, like glyco- 

 gen, in which they are held in reserve, or, for the time 

 being, placed beyond the reach of the cells. When, 

 however, the supply is reduced, the stored-up materials 

 reenter the blood and again become available to the 

 cells. By this means their rate of supply to the cells 

 is practically constant. 



We are now in a position to understand why carbohy- 

 drates, fats, and proteids are so well adapted to the needs 

 of the body, while other substances, like alcohol, which 



