1040 ABSORPTION 



Our search for the possible source of this glycogen leads us first 

 of all to mucin, which yields a considerable amount of carbohydrate, 

 but this substance does not play a significant part in metabolism. 

 Contrariwise, it has been proved that casein, which does not contain a 

 carbohydrate radicle, may be made to yield sugar when fed to animals 

 suffering from phloridzin glycosuria. Similar tests with different 

 amino-acids have given positive and negative results, although their 

 composition does not vary very considerably. Two of these, however, 

 have been proved to yield sugar, namely, alamin and aspartic acid. 

 In the former instance this conversion is not difficult chemically, be- 

 cause the substitution of HO in its molecule for NH2 gives lactic acid, 

 from which sugar may be obtained without much difficulty. Quite 

 similarly, if aspartic acid loses carbon dioxid, it is transformed into 

 lactic acid. Another substance, the conversion of which into carbo- 

 hydrate does not seem improbable, is glycerol. It may be concluded, 

 therefore, that the body possesses the power of forming its sugar from 

 the aforesaid substances, and possibly also -from other amino-acids, 

 although the chemistry of their conversion is not so obvious as in 

 the cases just cited. Under ordinary circumstances, however, the 

 body derives its glycogen directly from the carbohydrates of the food. 



The Utilization of Sugar. — In the cells of the liver a twofold 

 process is going on, namely, a conversion of the sugar into glycogen, 

 and a reconversion of this polysaccharide into circulating sugar under 

 the influence of an enzyme. We have seen that this circulating sugar 

 is changed in the pancreas into a form (colloid) which is more acceptable 

 to the tissue cells, i.e., one which they can burn up more readily than 

 ordinary glucose. Consequently, the glycogen of the liver serves as 

 a reserve material which is deposited here temporarily as an inert 

 polysaccharide. But the liver is not the only storehouse of sugar, 

 because it is also found in abundant amounts in the muscle tissue. 

 The difference between these two stores seems to be one of availability, 

 because if a muscle is suddenly cdUed upon to do extra work, it cannot 

 await the transfer of sugar from the liver. It is for this reason that 

 rapidly growing tissues invariably contain much glycogen which they 

 make use of in the course of their subsequent development. Thus, 

 while sugar is normally released by the liver into the blood stream, 

 the outlying depots are there for the purpose of serving the more im- 

 mediate needs of the body. 



It is a well-known fact that every contraction of muscle, whether 

 in the body or outside of it, consumes glucose. Thus, the normally 

 contracting heart necessitates about 4 mgr. of this substance per gram 

 of tissue in the course of 1 hour.^ Now, since the muscle tissue 

 forms about 42 per cent, of the body weight, its requirements in sugar 

 must be considerable. Moreover, since our body contains only 1.0 

 per cent, of carbohydrates, it will be seen that this foodstuff, contrary 



1 Starling and Knowlton, Jour, of Physiol., xlv, 1912, 146. 



