_Of the Composition of the Tissues and of the Secretions in Health and Disease._—Having described thus individually the constituents of the tissues, we shall now present such results as have been hitherto obtained as to the quantitative composition of the organized tissues formed by their reunion, their secretory products and morbid alterations. The skin of animals is a congeries of finely constructed organs, sensitive and secretory, imbedded in a peculiar tissue, which is one of those most yielding gelatine, whence the process of tanning skins. On the surface of the skin there is secreted a substance, which, though varying in anatomical structure and appearance exceedingly, as it forms the fine epidermis, the nails, the hair, etc., is yet throughout all their shapes identical in chemical character, and may be described as the same substance. The principal mass of hair is composed of the same substance as horn, but the color is due to an oil which may be extracted by ether. If by virtue of the sulphur contained in hair a solution of litharge in some limestone water blackens it, a solution of nitrate of silver will also blacken the hair, but by a deposition of the metal. The perspiration from the skin varies in nature according to the part of the body; it is generally acid, contains traces of albumen, fatty matter and the salts of the blood; it often contains, also, an odoriferous, volatile principle, characteristic of the animal by which it is secreted. _Cellular and Serous Tissues._—These tissues are constituted of gelatinous materials similar to that in the skin, and hence dissolve by boiling water, being converted into gelatine. In the natural condition of these membranes their surface is moistened by a watery liquid, which, accumulating in excessive quantity, gives rise to the dropsies of the cavities, or of the cellular tissues. This serum of the cavities is clear and colorless. The cells of the cellular tissues, in which fat is usually deposited, are often filled up by an albuminous material having considerable analogy with casein; it is thus that the diffused hardening of the cellular tissue and the local white tumors have their origin. _Of the Muscular Tissue._—From what has been already said of fibrine, it is evidently the essential element of the muscular tissues, and forms with water almost the whole of their parts. _Of the Bones._—In vertebrated animals with osseous skeletons the earthy material, in all cases, consists principally of phosphate of lime, with phosphate of magnesia, carbonate of lime and soda. By digesting a bone in dilute muriatic acid, all of these inorganic salts are removed, and the cartilage remains, preserving perfectly the form of the bone.