In the next place we must consider the purposes achieved by the class of fats or hydrocarbons in nutrition. Liebig's views with regard to this subject also have been shown to be erroneous. He considered the function of fats to be entirely respiratory, and that by combining with oxygen, admitted into the system in respiration, they were consumed in the production of heat, and that the completeness of this combustion depended on the amount of inspired oxygen. But it has been observed that when an exclusive diet of fat has been taken, there has been less fat metabolised and less oxygen absorbed than in fasting, and also that, in certain circumstances, the whole of the albumen in the food is metabolised in the body, and the fat is appropriated to increase the body-weight ; an inversion of the formerly assumed roles of hydrocarbons and albuminates. From which it would appear that, under certain conditions, fat is split up into simpler bodies with greater difficulty than albumen, and must not, therefore, be regarded as the same easily combustible substance in the organism that it is outside. 

It is not, then, through the direct action of oxygen that the non-nitrogenous foods any more than the nitrogenous ones are split up into simpler products, but by the agency of the cellular tissues, and the oxygen enters into these products "little by little." Indeed, under the influence of fat tissue-waste is lessened, and, therefore, less oxygen is taken into the system ; less oxygen being abstracted from the blood by the products of metabolism. 

We thus see that one of the great purposes served by fat in the food is to diminish albuminous metabolism, and it is, therefore, regarded as an "albumen-sparing" food. "If flesh alone be given, large quantities are required in order that nutrition and waste may balance one another, but if fat be added the demand for flesh is less." (Bauer.) 

But the fats have also an important relation in the body to the production of force and heat, to body-work and body-temperature. While, unlike the albuminates, the metabolism of hydrocarbons is independent of the amount taken in as food, it is notably affected by bodily exercise, which produces little effect on nitrogenous metabolism. The fats, therefore, undoubtedly minister to force-production, and undergo destruction and oxidation in the process ; so that the amount of carbonic acid given off" during exercise is much greater than during rest. 

External temperature also influences the meta- bolism of the hydrocarbons, and therefore the amount of carbonic acid excreted ; the lower the temperature, so long as that of the body itself is maintained, the greater the metabolism of non-nitrogenous foods, and the greater the amount of carbonic acid discharged from the body. This is one of the chief means of regulating the temperature of the body, and keeping it constant. 

When, however, the temperature of the body itself is disturbed, as in fever, then the higher the tempera- ture the greater the waste of the non-nitrogenous, as well as of the nitrogenous, constituents of the body, and the greater the excretion of carbonic acid, as well as of urea. 

It is probably through the nervous system that the exteiThil temperature influences the metabolic processes in the body, and especially through the peripheral sensory nerves.

 It would appear that albuminates and fats are, to a certain extent, opposed to one another in their action on the organism, as the former increase waste and promote oxidation, while the latter have the effect of diminishing them, and this they do prbably by affecting the metabolic activity of the cells of the tissues themselves. It is a matter of common observation that fat animals bear privation of food better than thin ones ; in the latter, their small store of fat is quickly consumed, and then the albumen is rapidly decomposed. It is for the same reason that corpulent persons, even on a very moderate amount of food, are apt to become still more corpulent. 

The influence of fat in the storage of albumen is exemplified by the fact that if 1,500 grammes of lean meat be given alone, it will be wholly decomposed ; but if 100 to 150 grammes of fat be added, then it will yield only 1,422 grammes of waste. It has also been shown that the balance of income and expenditure of albuminates, although the amount taken in the food may be very small, is readily established as soon as one adds a certain quantity of fat. A dog who took daily 1,200 grammes of lean meat was observed to be still losing some of the albuminous constituents of the body ; whereas, with only 500 grammes of flesh and 200 grammes of fat, the nutritive balance was rapidly re-established. The same has been observed in man. Rubncr found that an individual taking daily 1,435 grammes of meat, containing 48.8 grammes of nitrogen, lost by the kidneys 50.8 grammes of nitrogen ; whereas another taking meat and bread containing 23.5 grammes of nitrogen, to which were added 191 grammes of fat, only eliminated 19 grammes of nitrogen on the second day of the diet ; so that a small quantity of albumen, when combined with fat, is sufficient to maintain the albuminous structures of the body. As a practical conclusion from these considerations, we should note, that if we wish to increase the weight of the body and add to its con- stituents, we must not rely on an excess of albu- minates, as these given alone only lead to increased waste ; but if we combine fats with albuminates in proper proportions, an appreciable increase of both the nitrogenous and non-nitrogenous constituents of the body can be maintained for a considerable time. 

We see, then, how a proper use of fat economises the albuminous elements of food and checks the waste of the albuminous tissues. Fat enters into all the tissues. By its decomposition and oxidation it yields muscular force and heat, and it is therefore largely consumed in muscular exercise. By its capacity of being stored up in the body as adipose tissue, it provides a reserve store of force-producing and heat- generating material which can be utilised as required. 

The supporting influence of fat under great muscular fatigue is strongly maintained by Ebstei : and it is stated that the German Emperor, in the war of 1870, recognised this fact by requiring that each soldier should have served out to him daily 250 grammes of fat bacon!