Page:Popular Science Monthly Volume 23.djvu/392

378 appetite is fundamental to that of cookery. A healthy, unvitiated appetite is an index to the requirements of nutrition. Other illustrations of this will be presented as we proceed.

Another important constituent of animal food is gelatin or gelatine. It constitutes a large proportion of the whole hulk of the animal; it is, in fact, the main constituent of the animal tissues, the walls of the cells of which animals are built up being composed of gelatin. I will not here discuss the question of whether Haller's remark, "Dimidiurn corporis kuniani gluten est" ("Half of the human body is gelatin"), should or should not now, as Lehmann says, "be modified to the assertion that half of the solid parts of the animal body are convertible, by boiling with water, into gelatin." Lehmann and others give the name of "glutin" to the component of the animal tissue as it exists there, and gelatin to it when acted upon by boiling water. Others indicate this difference by naming the first "gelatin," and the second "gelatine."

The difference upon which these distinctions are based is directly connected with my present subject, as it is just the difference between the raw and the cooked material, which, as we shall presently see, consists mainly in solubility.

Even the original or raw gelatine varies materially in this respect. There is a decidedly practical difference between the solubility of the cell-walls of a young chicken and those of an old hen. The pleasant fiction which describes all the pretty gelatine preparations of the table as "calf's-foot jelly," is founded on the greater solubility of the juvenile hoof, as compared with that of the adult ox or horse, or to the parings of hides about to be used by the tanner. All these produce gelatine by boiling, the calves' feet with comparatively little boiling.

Besides these differences there are decided varieties, or, I might say, species of gelatine, having slight differences of chemical composition and chemical relations. There is chondrine, or cartilage gelatine, which is obtained by boiling the cartilages of the ribs, larynx, or joints for eighteen or twenty hours in water. Then there is fibroine, obtained by boiling spiders' webs and the silk of silk-worms or other caterpillars. These exist as a liquid inside the animal, which solidifies on exposure. The fibers of sponge contain this modification of gelatine.

Another kind is chitine, which constituted the animal food of St. John the Baptist, when he fed upon locusts and wild honey. It is the basis of the bodily structure of insects; of the spiral tubes which permeate them throughout, and are so wonderfully displayed when we examine insect anatomy by aid of the microscope, also of their intestinal canal, their external skeleton, scales, hairs, etc. It similarly forms the true skeleton and bodily framework of crabs, lobsters, shrimps, and other Crustacea, bearing the same relation to their shells, muscles, etc., that ordinary gelatine does to the bones and softer tissues of the vertebrata; it is "the bone of their bones, and the flesh of their flesh."