FIXED [FATTY] OILS, FATS AND WAXES The substances to be considered under this head divide them selves naturally into two large classes, viz., fatty (fixed) oils and fats on the one hand, and waxes on the other, the distinction be tween the two classes being based on a most important chemical difference. The fixed oils and fats consist essentially of glycerides, i.e., esters formed by the union of three molecules of fatty acids with one molecule of the trihydric alcohol glycerol (glycerine, q.v.).
In a class of glycerides known as the phosphatides (q.v.) or phospholipines one or more of the fatty acid radicles is replaced by a grouping containing nitrogen and phosphorus, as, for exam ple, lecithin.
The waxes consist of esters formed by the combination of one molecule of fatty acid with one molecule of a monohydric alco hol, such as cetyl alcohol, cholesterol, etc. Only in the case of the wax coccerin are two molecules of fatty acids combined with a dihydric alcohol. It must be pointed out that this distinction does not find ready expression in common parlance; thus Japan wax is a glyceride and should be more correctly termed Japan tallow, whereas sperm oil is, chemically speaking, a liquid wax. Although these two classes of substances have a number of physical proper ties in common it is better, for some purposes, to consider them under separate heads. The true chemical constitution of oils and fats was first expounded by the classical researches of Chevreul, embodied in his work, Recherches sur les corps gras d'origine ani male (1823, reprinted 1889).
Occurrence.—The oils and fats cannot be looked upon as definite chemical individuals, but as representatives of natural species which vary, although within narrow limits, according to the climate and soil in which the plants producing them are grown, or, in the case of animal fats, according to the climate, the race, the age of the animal and especially the food as well as the idio syncrasy of the individual animal. The oils and fats are dis tributed throughout the animal and vegetable kingdom, from the lowest organisms to the highest, and are found in almost all tis sues and organs. The vegetable oils and fats occur almost ex
clusively in the seeds and fruits; the waxes appear usually as thin films covering leaves and fruits. In animals the fats are enclosed mainly in the cellular tissues of the intestines and of the back, although a certain amount of fat is present in all the organs; in the lower animals fat is also stored in the liver, muscles, etc. There are a few liquid waxes known, occurring in the blubber and head cavity of sperm whales and stored in the stomach of certain arctic and antarctic birds (e.g., Australian mutton-bird [q.v.]) ; other waxes occur as insect secretions, while wool-wax is the natural grease found in sheep's wool.
Physiology of the Fats.—The most evident function of the fats in animal organisms is that of food-reserve, to supply, by subsequent oxidation, energy for the growing and working tissues. The storage of fats and oils in vegetable seeds can be similarly explained as food-reserve for the embryo; it is difficult, however, to account for the presence of oil in large quantities in the pericarp of fruits such as the olive and palm, which is probably destroyed before germination of the seed. The fats, and especially the waxes, can fulfil other valuable service by virtue of their physical proper ties, e.g., insolubility in water, and their chemical inertness. For instance, subcutaneous fat deposits protect the organism from cold; beeswax prevents dilution of the concentrated sugar solu tions of the comb by external moisture; the waxes, and in some cases the fats, secreted in the leaves of plants preserve the under lying tissues from loss or access of water. It is probable that the resistant powers of the tubercle bacillus are largely attributable to the protective effect of the wax coating in which it is encased; this waxy secretion is remarkably inert and resists the action of the usual hydrolysing measures. It is probable that the fats fulfil still other, though more recondite, essential functions, quite distinct from their office as fuel supply for the cells.