Bone consists of earthy salts deposited in a finely granular form throughout the substance of cartilage. By soaking a piece of bone in vinegar or other dilute acid, the earthy salts will be dissolved, the soft cartilage being left. But the structure of cartilage may be best observed by making thin sections of the gristle covering the ends of bones. It exhibits a bluish-white basis (Pl. IX. fig. 16), in which are imbedded numerous cells or _cartilage-corpuscles_, often undergoing cell-division. In some kinds of cartilage, the basis is composed of fibres. _Muscle._--On examining a piece of the red flesh of an animal under a low power, the mass will exhibit a number of coarse, parallel, longitudinal, dark lines (Pl. IX. fig. 20), the substance between these lines being marked with cross or transverse striæ, or lines, and with fine longitudinal lines. The coarse longitudinal lines indicate the intervals between bundles of slender fibres, or _fibril´læ_, of which muscle consists. The fibrillæ (fig. 20 _b_, _c_) are very difficult to separate; but when perfectly separated, they are seen to be exceedingly slender, and to consist of alternately light and dark portions in regular series. When the fibrillæ of the bundles are in close apposition, as in the natural muscle, the dark portions, being in the same lines, by their coincidence form the transverse striæ. The bundles into which they are combined are surrounded by a delicate skin or membrane, with a little cellular tissue. The structure of muscle may be observed in a piece of ham which has been soaked for a day or two in spirit of wine, the mounted needles being used to pick it to pieces. The above-mentioned transversely striated muscular fibre is that found in the voluntary muscles, or those under the influence of the will. But there are other muscles in animals which are involuntary, or not subject to the will; in these the fibrillar structure is absent, the muscular tissue consisting of simple elongated and nucleated cells. _Cel´lular tissue._--This fills the interstices between the other tissues and organs of animals, in the same manner that the vegetable parenchyma does those of plants. It is not, however, composed of cells, but of very fine, soft, colourless, and wavy fibres (Pl. VIII. fig. 20 _a_), aggregated into bundles, which interlace so as to leave spaces or aréolæ between them. The cellular or areolar tissue may be found in a piece of beef or mutton, in the intervals of the muscular fibres. _Skin._--The skin is composed of cellular tissue, its outer surface presenting a number of projecting blunt points, called _papil´læ_. It contains a large number of blood-vessels; and when the capillaries are well filled by injection with a coloured composition, it forms a beautiful microscopic object.