THE METABOLISM OF FAT 557 



tions and syntheses by assuming that the fat passes in the torm of 

 very fine droplets through the walls of the cells and of the capil- 

 laries. The reader will observe that we seem to be discussing again, 

 and almost in the same terms, the question of the absorption of fat 

 from the intestine. It is indeed at bottom the same question, and 

 it might be argued that by analogy it should receive the same solu- 

 tion. Analogy, however, is a dangerous guide in such matters, and 

 it is even more difficult to secure an unambiguous experimental test 

 of the manner in which the internal migration of fat is accomplished 

 than to secure the like for its absorption from the digestive tube. 

 , As to the ultimate fate of the fat, from whatever source it may \ 

 / be derived, our knowledge may be compressed into very few sen- \ 

 tences : Sooner or later it is split and oxidized to carbon dioxide and 

 water, its energy being converted into heat or, directly or indirectly, into 

 mechanical or other functional work ; some of the fat absorbed from the 

 intestine rapidly undergoes this change without entering the fat-cells of 

 the adipose tissue. A portion of the fat may be changed into carbo- 

 hydrates. This has been proved for the glycerin component ; its possi- 

 bility must be admitted for the fatty acids, but proof has not yet been given. ^ 



Of the intermediate stages by which the fatty acids are degraded 

 into the simple end products but little is surely known. Included 

 among these stages must be the compounds with which the forma- 

 tion of the acetone bodies (p. 553) starts, if and in so far as their 

 formation is a normal event which is merely unveiled by the dis- 

 turbance of the ordinary course of the metabolism in diabetes. 

 Among these intermediate stages must also be included, it is to be 

 supposed, the compounds, whatever they may be, which act as 

 connecting links between the currents of fatty acid and of carbo- 

 hydrate metabolism, and with which the transformation of fatty 

 acids into carbo-hydrates commences, if this occurs at all. 



According to the observations of Knoop, the saturated as well as 

 some of the other series of fatty acids when oxidized decompose in a very 

 characteristic way. As already remarked, these acids are made up of a 

 larger or smaller number of CH 2 groups forming a chain which at one 

 end terminates with a carboxyl (COOH) group, and at the other with a 

 CH 3 group. The carbon atoms in the chain are designated by Greek 

 letters, a, /3, etc., the a position being that next the carboxyl group, 

 the /3 position one remove from the carboxyl group, and so on. Accord- 

 ing to Knoop, the oxidation of the fatty acid chain takes place in such 

 a way that the chain is shortened by the cutting off from the carboxyl 

 end the a CH 2 group along with the carboxyl group, while in place of the 

 CH 2 group there is left a carbcxyl group, an operation which might 

 be fancifully compared to the naval manoeuvre of breaking the enemy's 

 line. Thus from caproic acid CH 3 .CH 2 .CH 2 .CH 2 . j CH t .O >H, we 



get by oxidation butyric acid, CH 3 .CH 2 .CH a .C( 'H, carbon dioxide 



and water. It appears that the oxidation proceeds in two stages, the 

 hydrogen of the /3 group being first oxidized with formation of an 



