THE ANIMAL BODY DIGESTION METABOLISM 35 



possessed by the liver alone, but by the tissues of the body generally, 

 especially the muscles. When work is being done the glycogen in the 

 muscles is first drawn upon to furnish glucose, and after this store has 

 been exhausted, the glycogen in the liver furnishes the needed glucose. 



61. Absorption of proteins. We have previously learned how the pro- 

 tein in the food is digested by pepsin, trypsin, and erepsin, and split into 

 ammo acids, the "building stones" of the proteins. These amino acids 

 are absorbed by the villi of the small intestine. It was formerly be- 

 lieved that they were joined together while still within the intestinal 

 walls, forming the complex proteins of the blood called serum albumin 

 and serum globulin. Thru refinement of experimental methods, Van 

 Slyke, 4 and Folin and Denis 5 have been able to prove that the amino 

 acids are not necessarily thus built into blood proteins in the intestinal 

 wall, but that they may pass into the blood stream without being united. 

 They are then carried into the general circulation, and from the blood 

 stream each of the various tissues of the body muscles, organs, etc. 

 absorbs a certain amount of the amino acids for growth, or the repair 

 of the daily waste of protein matter. 



Mineral matter is absorbed by the villi of the small intestine, and 

 water is taken up all along the digestive tract, from the stomach to the 

 large intestine. 



62. Distribution of absorbed nutrients. We have seen that the digested 

 fats, which are to nourish the body, are poured into the blood current by 

 way of the lymphatics, while the sugars, the amino acids, and the mineral 

 matter enter the blood directly thru the capillaries and veins. The vari- 

 ous nutrients are carried in the blood thru the circulation to the capil- 

 laries. These are so constructed that, when the blood finally reaches 

 them, the nutritive substances it carries pass thru their walls and are 

 mingled with lymph that bathes the myriad body cells. In this manner 

 all the nutrients, having been especially prepared and transported, are 

 available for the nourishment of every portion of the body. 



63. Use of the absorbed nutrients. The absorbed nutrients, thus 

 transferred to all the tissues of the body, may be oxidized, or burned, to 

 warm the animal, or to produce energy to carry on the vital processes 

 and to perform work, as shown in the following chapters. In case more 

 nutrients are supplied than are required for these purposes, the excess 

 may be transformed into body tissue proper, as shown in Chapter V. 

 The sugars may be converted into fats and stored as body fat, as may 

 also the fats derived directly from the food fats. The amino acids may 

 be built up into body protein or, if not needed for this purpose, a portion 

 of their carbon, hydrogen, and oxygen may be converted into fat, while 

 the nitrogen is excreted, chiefly in the form of urea. The highest use 

 of the proteins, however, is the formation of nitrogenous tissues the 

 muscles, brain, nerves, skin, hair, and various organs of the body. 



4 Jour. Biol. Chem., 12, 1912, 399-410; 16, 1913, 187-233. 

 "Jour. Biol. Chem., 11, 1912, 87-95; 12, 1912, 141-162. 



