lean animal the average proportions of the various constituents will be 54 per cent. of water, 25½ per cent. dry fat, 17 per cent. of dry nitrogenous substances, and 3½ per cent. of mineral matter. In the following table these proportions are set forth.

SECTION III.

USE OF FAT IN THE ANIMAL ECONOMY.

As fat forms so large a portion of the body, it is evident that the part it plays in the animal economy must be a most important one. The general opinion which prevails amongst scientific men as to its physiological functions was originated by the celebrated Liebig. According to his theory, the food of animals includes two distinct kinds of substances—plastic

4 and non-plastic. The plastic materials are composed of carbon, hydrogen, oxygen, nitrogen, and a little sulphur and phosphorus. Albumen, fibrine, and casein are plastic elements of nutrition; they form the lean flesh, or muscles, the membranes, and cartilages, the gelatine of the bones, the skin, the hair, and, in short, every part of the body which contains nitrogen. The non-plastic elements of nutrition include fat, oil, starch, sugar, gum, and certain constituents of fruits, such as pectine.

All non-plastic substances—and of each kind there are numerous varieties—are capable of conversion, in the animal mechanism, into fat and oil. The non-plastic food substances do not contain nitrogen, hence they are commonly termed non-nitrogenous elements. The oily and fatty matters contain a large proportion of carbon, their next most abundant component is hydrogen, and they contain but little oxygen. Unlike the plastic elements, they are—except the fats of the brain and nervous tissue—altogether destitute of sulphur and phosphorus. The starchy, saccharine, and gummy substances are composed of the same elements as the fatty bodies, but they contain a higher proportion of oxygen. According to Liebig, fat is used in the animal economy as a source of internal heat. We all know that it is a most combustible body, and that during its inflammation the most intense heat is developed. It is less evident, but not less true, that heat is evolved during its slow oxidation, or decay.

The more rapidly a body burns, the greater is the amount of heat evolved by it in a given time; but the total amount of heat developed by a specific weight of the body is the same, whether the combustion takes place rapidly or slowly. An experiment performed with phosphorus illustrates the case perfectly. If we burned two pieces of equal weight, the one in oxygen, the other in atmospheric air, we should find that the former would emit a light five times as brilliant as