PHYSIOLOGY OF DIGESTION 65 



circulation and carried to the liver. Most of the portion carried 

 to the liver is there changed back from sugar to a starch called 

 glycogen and stored in the liver as such. As the blood loses its 

 sugar, it is resupplied from the liver. Thus the liver becomes 

 a storehouse for surplus sugar and at the same time serves to 

 maintain a uniform percentage of sugar in the blood. Some 

 of tiie sugar may be changed into fat. The fat and sugar are 

 ultimately oxidized, liberating mechanical energy, heat, CO2, 

 and water. The two latter are excreted by the skin, lungs and 

 kidneys. 



Proteids. — These are not affected by the saliva. But they are 

 changed to soluble amino acids, intermediate products, and 

 blood proteids by the gastric and pancreatic juices, and by the 

 epithelial cells which line the intestine and through which ab- 

 sori)tion takes place. The digested proteids are then absorbed 

 into the blood which goes to the liver, and are ultimately dis- 

 tril)uted to tissue cells to replace worn out parts or to build up 

 new parts as in growing animals. 



It has been held that proteid nutrients are especially neces- 

 sary to repair tissues that have been worn out by mechanical 

 work; but there is good reason to suppose that this has been 

 overestimated in the past, and that carbohydrates have much to 

 do with the ultimate source of physical power. 



The proteid products are finally oxidized, and give heat and 

 mechanical energy. They are excreted from the body as CO2, 

 urea and water. The urea is excreted mainly by the kidneys, 

 and the water by all the excretory organs. 



Fats. — These are digested mainly by the pancreatic fluid. 

 They are not acted on by either the saliva, or the gastric juice, 

 and they are but slightly affected by the bile, although the 

 latter is indirectly important in the digestion and absorption 

 of fats. The pancreatic fluid emulsifies a portion which reaches 

 the laeteals in this emulsified condition. The remainder is di- 

 gested by separating the glycerin from the fatty acid (fat con- 

 sists of a fatty acid plus glycerin). Glycerin is readily soluble 

 and easily absorbed. The fatty acid unites with alkaline ma- 

 terial from the bile and pancreas to form soluble soap, and thus 

 the remainder of the fat is absorbed. Distribution is made 

 by the blood current and finally, assimilation takes place, 

 i.e., building tissue cells, and then oxidation. Fat may 

 either be deposited into and become a part of tlie cell bodies 



