378 THE POPULAR SCIENCE MONTHLY. 



appetite is fundamental to that of cookery. A healthy, unvitiated ap- 

 petite 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 as- 

 sertion 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 " gela- 

 tine." 



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, con- 

 sists 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 juve- 

 nile hoof, as compared with that of the adult ox or horse, or to the par- 

 ings of hides about to be used by the tanner. All these produce gela- 

 tine 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 composi- 

 tion 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, ob- 

 tained by boiling spiders' webs and the silk of silk-worms or other cat- 

 erpillars. 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 Avhich per- 

 meate them throughout, and are so wonderfully displayed when we 

 examine insect anatomy by aid of the microscope, also of their intesti- 

 nal 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 ver- 

 tebrata ; it is "the bone of their bones, and the flesh of their flesh." 



