CHE3IISTBY OF DIGESTION AND NUTRITION. 35] 



Formation of Pat in the Body. — The origin of body-fat has always been 

 an interesting problem to physiologists. Naturally, the first supposition made 

 was that it comes directly from the fat of the food. According to this view, 

 a certain proportion of the fat of the food was supposed to be deposited directly 

 in the cells of adipose tissue, and in this way all our supply of fat originated. 

 This theory was soon disproved. It was shown, especially upon cows and pigs, 

 that the amount of fat formed in the body within a given time, including the 

 fat of milk in the case of the cow, might be far in excess of the total amount 

 of fat taken in the food during the same period, thus demonstrating that a cer- 

 tain proportion at least of the body-fat must have some other origin. More- 

 over, the genesis of the fat-droplets in fat-cells, as studied under the microscope, 

 did not agree with the old view ; and there was the further fact that each animal 

 has its own peculiar kind of fat; as Liebig says, "In hay or the other fodder 

 of oxen no beef-suet exists, and no hog's lard can be found in the potato refuse 

 given to swine." In fact, the evidence was so conclusive against this theory that 

 physiologists for a time were led to adopt the opposite view that no fat at all can 

 be obtained directly from the fat of the food. However, it has now been shown 

 that under certain conditions fat may be deposited directly in the tissues from 

 the fat of food. Lebedeff, and afterward Munk, proved that if a dog is first 

 starved until the reserve supply of fat in the body is practically used up, and 

 it is then fed richly upon foreign fats, such as rape-seed oil, linseed oil, or 

 mutton tallow, it will again lay on fat, and some of the foreign fat may he 

 detected in its body. The conditions necessary to be fulfilled in order to get 

 this result make it probable that under normal conditions none of the fat of 

 the body is derived directly from the fat of the food. On the contrary, the 

 fat of the food is completely oxidized, and our body-fat is normally o in- 

 structed anew from either proteids or carbohydrates. As to its origin from 

 proteid, Voit has devoted numerous researches to the purpose of demon- 

 strating that this is the main source of body-fat. His belief is that in the 

 course of metabolism the proteid molecule undergoes a cleavage, with the for- 

 mation of a nitrogenous and a non-nitrogenous part. The former, after further 

 changes, is eliminated in the form of urea, etc. ; the latter may be converted 

 into fat, or possibly into glycogen. The theoretical maximum of fat which 

 can arise in this way is 51.5 per cent, of the entire amount of proteid. Voit 

 attempted to demonstrate this theory by actual experiments. lb' showed that 

 dogs fed upon large amounts of lean meat did not give off as much carbon in 

 the excreta as they received in the food. The excess of carbon must have been 

 retained in the body, and, in all probability, in the form of fat. As corrob- 

 orative evidence he cites the apparently direct conversion of proteid material 

 into fat in such cases as the formation of lat-droplets in the fat-cells or cells 

 of the mammary glands, and in muscle-fibres and liver-cells undergoing fatty 

 degeneration ; but evidence of this latter character is not conclusive, since we 

 have no immediate proof that the faf arises directly from the proteid material 

 in the cells. Voit's experimental evidence has been questioned recently by 

 Pfliiger, his criticisms being directed mainly toward the calculations involved 



