450 THE HUMAN BODY 



as to set their nitrogen-containing parts off from their more readily 

 oxidizable carbon and hydrogen-containing portions. We may 

 readily suppose, from what has been assumed as to the small 

 actual protein requirement of the tissues, that the amount of 

 protein absorbed as such into the portal vein at any one time is 

 too small to be detected, in the presence of the normally rather 

 high protein content of blood. If, however, it be true that the 

 protein which is destined to serve as fuel is split in such fashion 

 as to separate the nitrogenous from the non-nitrogenous portion 

 we ought to be able to find in the portal blood some of these split 

 products. Very fortunately for the theory this we are able to do. 

 It has been shown that the blood of the portal vein contains am- 

 monia compounds which have been absorbed from the intestine. 

 That proteins can be so split as to yield a large part of their nitro- 

 gen as ammonia has long been known, so that this observation 

 accords perfectly with the theory. Special non-nitrogenous deriv- 

 atives of protein have not been demonstrated in portal blood, but 

 it is assumed that they are so similar to the carbohydrates in it 

 as to be indistinguishable as separate substances; and that they 

 undergo conversion into glycogen in the liver in common with 

 the carbohydrates. 



The Absorption of Fats. The result of fat digestion is to split 

 the fats to fatty acid and glycerin. It is believed that they are 

 taken up by the cells of the intestinal lining partly in this form; 

 but not wholly so, since free fatty acid in the presence of free 

 alkali, such as is furnished by the bile and pancreatic juice, reacts 

 with the alkali to form soap. That there is in the small intestine 

 a certain amount of soap formation cannot be doubted. The ad- 

 vantage of soap formation is one of increased solubility; fatty' 

 acids are insoluble in water, soap quite soluble. There is reason 

 to believe, however, that only part of the fatty acid is combined 

 into soap, and that the remainder is absorbed, as stated above, 

 as fatty acid. This direct fatty acid absorption seems to be ef- 

 fected largely through the agency of the bile. It is known that 

 fatty acids are soluble in bile, and can thus be brought in solution 

 into contact with the absorbing cells; and a very common observa- 

 tion of physicians is that stoppage of the flow of bile into the in- 

 testine, as by occlusion of the bile-duct, is followed by an almost 

 complete failure of fat absorption. The glycerin part of the de- 



