43 8 ABSORPTION 



is accomplished, the most fascinating theory is that which attributes 

 it to the reversed action of lipase, possibly the very same lipase as 

 originally split it up in the intestine. The reversibility of the action 

 of various enzymes under changed conditions, especially changes in 

 the relative concentration of the bodies concerned in the reaction, 

 has been previously mentioned. It has been shown, e.g., that the 

 pancreas, intestinal mucous membrane, lymph glands, etc., and 

 even cell-free extracts of these organs have the power of synthesizing 

 the ester ethyl butyrate from butyric acid and ethyl alcohol 

 (p. 332), as well as the power of decomposing the ester into the 

 fatty acid and the alcohol. Moore, however, states that in the 

 case of ordinary fats the synthesis takes place in the intestinal wall 

 only in situ, and while the circulation is going on. In the intestinal 

 mucosa the greater part of the fatty acid is already combined with 

 glycerin as neutral fat, although considerable quantities of free fatty 

 acid are also present. In the lymph coming directly from the 

 mesenteric glands practically the whole of the fatty acids are in 

 the form of neutral fat. 



An additional, and in some respects even more remarkable, illus- 

 tration of the synthesizing powers of the intestinal wall is the dis- 

 covery of Munk, already referred to (p. 433), that fatty acids given 

 by the mouth appear in the lymph of the thoracic duct as neutral 

 fats, having somewhere or other, in all probability on their way 

 through the epithelium of the gut, been combined with glycerin. 



Since, however, the amount of neutral fat recovered from the 

 thoracic duct is not equivalent to more than one-third of the fatty 

 acids given, it has been suggested that this synthesis of fat is only 

 apparent, and that the whole of the fat which appears in the chyle 

 after a meal of fatty acids comes from the fat excreted into the 

 intestine (Frank), which is increased when fatty acids are given by 

 the mouth. But the suggestion is more ingenious than the evidence 

 advanced in its support is convincing. And, as we have seen 

 (p. 437), a part of the deficit may be accounted for by absorption 

 directly into the bloodvessels. 



In concluding our review of the absorption of fat, certain general 

 considerations which have a close relation to the question may be 

 alluded to. There is some reason to think that the lipases are 

 enzymes less finely adjusted to minute differences in the structure 

 of the fats on which they act than other digestive ferments e.g., 

 maltase or lactase, to details in the chemical structure of their 

 substrates. If this be so, a very few lipases, or even a single one, 

 may suffice to accomplish all the enzymatic changes which occur 

 in the fats both in the lumen of the intestine and in all the various 

 tissue cells. At the same time the possible variation in those decom- 

 position products which constitute the ' building-stones ' of the fats 

 is less than in the case of, say, the proteins. Two consequences 



