290 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and H 2 O and a corresponding amount of energy must be liberated. Speak- 

 ing generally, then, the essential nutritive value of the fats is that they furnish 

 energy to the body, and, from a chemical standpoint, they must contain more 

 available energy, weight for weight, than the proteids or the carbohydrates 

 (see p. 303). In a well-nourished animal a large amount of fat is found 

 normally in the adipose tissues, particularly in the so-called "panniculus 

 adiposus" beneath the skin. Physiologically, this body- fat is to be regarded 

 as a reserve supply of nourishment. When food is eaten and absorbed in 

 excess of the actual metabolic power of the body, the excess is stored in the 

 adipose tissue as fat, to be drawn upon in case of need as, for instance, 

 during partial or complete starvation. A starving animal, after its small 

 supply of glycogen is exhausted, lives entirely upon body-proteids and fats ; 

 the larger the supply of fat, the more effectively will the proteid tissues be 

 protected from destruction. In accordance with this fact, it has been shown 

 that when subjected to complete starvation a fat animal will survive longer 

 than a lean one. Our supply of fat is called upon not only during complete 

 abstention from food, but also whenever the diet is insufficient to cover the 

 oxidations of the body, as in deficient food, sickness, etc. 



Formation of Fat 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 be 

 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 con- 

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



