ABSORPTION, STORAGE, AND ASSIMILATION 177 



A Second Purpose of Digestion. Comparing the digestive changes 

 with those of absorption, it is found that they are of a directly opposite 

 nature ; that while digestion is a process of tearing down, or separating, 

 one which reduces the food to a more finely divided condition 

 there is in absorption a process of building up. From the compara- 

 tively simple compounds formed by digestion, there are formed during 

 absorption the more complex compounds of the blood. The one excep- 

 tion is dextrose, which is a simple sugar; but even this is combined in 

 the liver and the muscles to form the more complex compound known 

 as glycogen. (See Methods of Storage, below.) These facts have sug- 

 gested a second purpose of digestion that of reducing foods to forms 

 sufficiently simple to enable the body to construct out of them the more 

 complex materials that it needs. Evidence that digestion serves such a 

 purpose is found in the fact that both proteids and carbohydrates are 

 reduced to a simpler form than is necessary for dissolving them. 1 



The Storage of Nutriment. For some time after the 

 taking of a meal, food materials are being absorbed more 

 rapidly than they can be used by the cells. Following 

 this is an interval when the body is taking no food, but 

 during which the cells must be supplied with nourishment. 

 It also happens that the total amount of food absorbed 

 during a long interval may be in excess of the needs of 

 the cells, during that time; and it is always possible, as 

 in disease, that the quantity absorbed is not equal to that 

 consumed. To provide against emergencies, and to keep 

 up a uniform supply of food to the cells, it is necessary 

 that the body store up nutrients in excess of its needs. 



Methods of Storage. The general plan of storage 

 varies with the different nutrients as follows : 



i. The carbohydrates are stored in the form of glycogen. 

 This, as already stated (page 120), is a substance closely 

 resembling starch. It is stored in the cells of both the 



1 The soluble double sugars (maltose, milk sugar, and cane sugar) are reduced 

 to the simple sugars (dextrose and levulose). Furthermore the action on the 

 proteids does not stop with the production of peptones and proteoses, but these in 

 turn are still further reduced. 



