Bones are composed of two kinds of substance--animal, which makes the bone tough and flexible; earthy, which makes it hard and fragile. In young animals the animal matter forms about one-half of the bone substance; in the adult, it diminishes to about a third, while in old animals it is still less; hence the bones of very old animals are brittle, more liable to fracture and harder to mend. Bones in a six-year-old horse contained, Phosphate of Lime, 54.37 per cent; Carbonate of Lime, 12.00 per cent; Phosphate of Magnesia, 1.83 per cent; Soluble Salts, 0.70 per cent, or mineral matter, 68.90 per cent. While they contain Cartilage, 27.99 per cent; Fat, etc., 3.11 per cent, or animal matter 31.10 per cent. In bone tissue there are two modifications of texture, the _compact_ and the _cancellated_. The former--hard, dense, and ivory-like, is always situated externally; the latter porous and spongy lies within. Although the compact tissue appears uniformly dense, and destitute of porosity, yet, if we transversely sectate the shaft of a long bone, and examine it under the microscope, by transmitted light, it is found to contain numerous round openings. These are called _Haversian canals_. They transmit bloodvessels, and run in a longitudinal or slightly oblique direction, opening on either the outer or inner surface of the bone. The external (outer) surface of every bone is covered by a tough, fibrous, inelastic membrane called periosteum, which can be seen by examining the bone of an animal which has recently died. The only exception to this is at the joints where one bone articulates with another, and where a tendon or muscle plays over a bone; here we find its place taken by articular cartilage. By its strength it sometimes retains bones in contact after an oblique fracture; in the young, it is thicker and more vascular than in the adult. Blood-vessels which penetrate the periosteum pass directly to the bone; the outer surface of the bone is always studded with numerous _foramina_ through which these enter. The periosteum owing to its inelasticity, is, when inflamed, the seat of intense pain; and should any part of it be stripped off, there is every probability of the denuded bone dying and separating. CONTENTS OF BONE.--Red marrow is found in the extremities or near the ends of bones, white marrow is found in the shaft.