DIGESTION AND ABSORPTION IN THE INTESTINES. 805 



portal vein and passes through the liver before reaching the general 

 circulation. The liver holds back more or less of the fat taking 

 this route, as it is found that during absorption the liver cells show 

 an accumulation of fat droplets in their interior.* The amount of 

 fat that may be absorbed from the intestines varies with the 

 nature of the fat. Experiments show that the more fluid fats, such 

 as olive oil, are absorbed more completely, that is, less is lost 

 in the feces than in the case of the more solid fats. Compara- 

 tive experiments have given such results as the following: Olive 

 oil absorption, 97.7 per cent.; goose and pork fat, 97.5 per 

 cent.; mutton fat, 90 to 92.5 per cent.; spermaceti, 15 per cent. 

 The amount of fat that may be lost in the feces varies also with 

 other conditions. If, for instance, an excess is taken with the 

 food, or if the bile flow is diminished or suppressed, the percentage 

 in the feces is increased. The usual amount of fat allowed as a 

 maximum in dietaries is from 100 to 120 gm. daily. 



Absorption of Proteins. Most of the experimental work on 

 record shows that the digested proteins are absorbed by the blood- 

 vessels of the villi, although after excessive feeding of protein a 

 portion may be taken up also in the lymphatics, f This accepted 

 belief rests upon two facts: First (Schmidt-Mulheim), if the thoracic 

 duct (and right lymphatic duct) is ligated, so as to shut off the lym- 

 phatic circulation, an animal will absorb and metabolize the usual 

 amount of protein as is indicated by the urea excreted during the pe- 

 riod. Second (Munk), if a fistula of the thoracic duct is established 

 and the total lymph flow from the intestines is collected during 

 the period of absorption after a diet of protein, it is found that there 

 is no increase in the quantity of the lymph or in its protein contents. 

 The form in which the digested protein enters the blood has long 

 been a matter of controversy. J On the view that we have adopted, 

 namely, that the protein of the food is split into its constituent 

 amino-acids by the successive action of the pepsin, trypsin, and 

 erepsin, we should expect to find these amino-acids in the blood, 

 unless, indeed, they are again synthesized to protein while passing 

 through the intestinal wall. It is only in recent years that meth- 

 ods have been devised for the recognition of amino-acids in such a 

 liquid as blood. The application of these methods tends to sup- 

 port the view that the amino-acids enter the blood as such without 

 undergoing synthesis. Alanin and valin have been obtained from 

 the blood in crystalline form, and histidin has been recognized by its 



* See Frank, "Archiv f. Physiologic," 1892, 497, and 1894, 297. 



t See Mendel, "American Journal of Physiology," 2, 137, 1899. 



j See especially Van Slyke and Meyer, "Journal of Biological Chemistry," 

 16, 197, 1913; also pp. 213 and 231. 



$ Folin and Denis, "Journal of Biological Chemistry," 11-87 and 161, 

 1912; and Van Slyke, ibid., 12, 399, 1912. 



