1046 ABSORPTION 



some of this oxygen will be made available for other oxidations, so that 

 the animal need not take in so large an amount by respiration. In 

 addition to this process of deoxidation, other changes are effected, 

 such as the reduction of glucose into two molecules of lactic acid which 

 in turn is converted into aldehyde and formic acid. By polymeriza- 

 tion, the aldehyde may then be changed into aldol which yields buty- 

 ric acid on oxidation or by transposition of its oxygen. ^ 



The Utilization of the Fats. — The final product of the metabolism 

 of the fats is carbon dioxid and water, and their chief function to supply 

 energy. This being the case, the body holds a considerable portion 

 of this substance in reserve as a deposit in its different storehouses. 

 Among the latter might be mentioned the liver, the tissues, and such 

 special structures as the panniculus adiposus in the deep skin, the omen- 

 tum, and retroperitoneal spaces. Any excess is stored in these places 

 to be drawn upon later on when needed. Thus, fat serves as an addi- 

 tional protection to the proteins, being itself safeguarded by the carbohy- 

 drates. It presents, however, different characteristics in accordance 

 with its origin and place of deposition. The ordinary depot-fat, for 

 example, yields 95 per cent, of its total weight as fatty acids, while the 

 tissue-fat yields only 60 per cent. This might imply that the former 

 is neutral fat, while the latter is combined into lecithin and phos- 

 pholipins. In the liver, the character of the fat varies with the inten- 

 sity of the metabolism of this organ, being more like the fat of the tissue 

 during its periods of relative quiescence and more like that of depot-fat 

 during its periods of activity. It is also apparent that the amount of 

 fat which may be stored in this way is almost unlimited, contrary to 

 glycogen which at best cannot be stored in much greater quantities 

 than 300 grm., i.e., 150 grm. in the liver and 150 grm. in the muscles 

 and other tissues. 



This depot-fat is mobilized and transported to the active tissue 

 whenever the latter has used up its own store of energy-yielding 

 material, and obviously, this mobilization necessitates its conversion 

 into fatty acids and glycerin, which products again give rise to neutral 

 fat in the blood. No doubt, the chief seat of these oxidations is the 

 muscle-tissue itself, and principally the cardiac and skeletal muscles. 

 In the former, for example, enough fat is stored up to last for 6 or 

 7 hours. The intake then being insufficient to cover the outgo, 

 all the available stored fat is drawn upon. Thus, a starving animal 

 first exhausts its relatively small store of glycogen and then its depot- 

 fat to the extent of 90 per cent, of the energy required. Consequently, 

 a fat animal is able to survive complete abstinence from food much 

 longer than a lean one. Besides this important function as a source 

 of energy, the body-fat also serves as a factor in regulating the body- 

 temperature by preventing an undue heat-dissipation, and lastly, as a 

 factor in protecting delicate structures from mechanical injury. 



The question whether the liver possesses a special influence upon 



^Leathes, The Fats, Monogr. in Bioch., Longmans, Green and Co. , 1912. 



