*Skeleton Tissues.*—The tissues employed in the construction of the skeleton are the osseous, the cartilaginous, and the connective tissues. These are known as the supporting tissues of the body. They form the bones, supply the elastic pads at the ends of the bones, and furnish strong bands, called ligaments, for fastening the bones together. The skeleton forms about 16 per cent of the weight of the body. Its tissues, being of a more durable nature than the rest of the body, do not so readily decay. Especially is this true of the osseous tissue, which may be preserved indefinitely, after removal from the body, by simply keeping it dry. *The Bones.*—The separate units, or parts, of which the skeleton is constructed are called bones. They are the hard structures that can be felt in all parts of the body, and they comprise nearly the entire amount of material found in the prepared skeleton. As usually estimated, the bones are 208 in number. They vary greatly in size and shape in different parts of the body. *Composition and Properties of Bones.*—The most noticeable and important properties of the bones are those of hardness, stiffness, and toughness. Upon these properties the uses of the bones depend. These properties may, in turn, be shown to depend upon the presence in osseous tissue of two essentially different kinds of substance, known as the _animal matter_ and the _mineral matter_. If a bone is soaked in an acid, the mineral matter is dissolved out, and as a result it loses its properties of hardness and stiffness. (See Practical Work.) This is because the mineral matter supplies these properties, being composed of substances which are hard and closely resemble certain kinds of rock. The chief materials forming the mineral matter are calcium phosphate and calcium carbonate. On the other hand, burning a bone destroys the animal matter. When this is done the bone loses its toughness, and becomes quite brittle. The property of toughness is, therefore, supplied by the animal matter. This consists mainly of a substance called _ossein_, which may be dissolved out of the bones by boiling them. Separated from the bones it is known as _gelatine_. The blood vessels and nerves in the bones, and the protoplasm of the bone cells, are also counted in with the animal matter. [Fig. 93] Fig. 93—*Section of a long bone* (_tibia_), showing the gross structure. If a dry bone from a full-grown, but not old, animal be weighed before and after being burned, it is found to lose about one third of its weight. From this we may conclude that about one third of the bone by weight is animal matter and two thirds is mineral matter. This proportion, however, varies with age, the mineral matter increasing with advance of years.