ASSIMILATION 89 



fluid portion of the blood of an animal be introduced into its 

 digestive cavity, this blood serum will very soon be absorbed into 

 the circulating fluid, which must have the same concentration. 

 It is, therefore, evident that factors other than osmosis and dif- 

 fusion must be considered. In the absorption of fat, it is clear 

 that the cells of the mucous membrane are not passive. The 

 fact that the glycerol and fatty acids are resynthesized is evi- 

 dence of protoplasmic activity. Indications of such activity are 

 not so obvious in the absorption of other substances, but it must 

 not be forgotten that in protoplasm we are dealing with a complex 

 colloidal system about which we are still seeking knowledge. 



The enormous surfaces present in colloidal systems make pos- 

 sible a great deal of adsorption, a well-known physical principle 

 which is responsible for the use of finely divided charcoal in gas- 

 masks. The gases in passing through the charcoal are adsorbed 

 on the surface of the finely divided particles. A consideration of 

 this phenomenon, along with other properties of colloidal systems, 

 is engaging the attention of some investigators who seek to explain 

 the physico-chemical basis for absorption. It is safe to say that no 

 explanation will be found satisfactory that docs not take into 

 consideration the metabohsm and organization of the cells of 

 the mucous membrane. 



Fate of the Nutrients Absorbed from the Digestive Tract. — The fat 

 absorbed into the Ijinph vessels in the submucosa passes, in man, by 

 way of the lacteals running in the mesentery to the thoracic duct 

 which empties into the left jugular vein. From there the finely emul- 

 sified fat is carried in the blood, and with the amino acids, simple 

 sugars, water, salts, and vitamins is distributed to all the cells in the 

 body by way of capillary networks. The veins carrying blood from 

 the digestive tract unite to form the hepatic portal vein, which emp- 

 ties into the capillary system of the liver. Here the greater part of 

 the simple sugars absorbed from the intestine leave the blood and 

 are synthesized in the liver cells to glycogen, a complex carbo- 

 hydrate known as animal starch. As glycogen, carbohydrates are 

 stored in the liver, and to a lesser extent in the muscles of the body. 

 According to the needs of other cells of the body, glycogen is con- 

 verted into glucose, passed into the blood stream, and distributed. 

 Interference with this control of the amount of sugar in the blood 

 stream results in abnormal conditions, of which diabetes is the 

 best known. The nature of this control will be discussed in a 



