The whole of the bones are composed of nearly the same materials, namely, amindial.or earthy substance, chiefly lime, and an animai substance called gelatine. Each bone is surrounded by a membranous covering, which contains numerous blood-vessels. On the outside of the bones are small openings, which are the mouths of tubes having canals connected with them, and which run through the bones. The blood-vessels of the membranous covering enter these openings, and so convey the blood into the tubes and canals, by means of which it is distributed through the bones, thus giving them nourishment, and contributing to their growth, solidity, and strength.

The bones of the limbs are hollow, the cavity being filled with a substance called marrow.

The ends of bones are covered with a gristly substance called cartilage, which, together with the oil or synovia secreted in every joint, prevents them from being injured by the constant friction to which they are exposed.

The principal parts of the skeleton are the head, the trunk, and the limbs.

The Head.

The skull, which contains the brain, is fixed at the top of the spine, or bones of the back. The bones of the skull are united together in a very peculiar manner. The edges of the bones

contain notches resembling the teeth of a saw. These enter and grow into each other; and as the bones are thicker at these joints than in other parts of the skull, they thus become strongly locked together. The bone of the forehead, or front part of the skull, is, unlike that of the lower animals, so formed as to convey an idea of power and dignity. The bones which form those parts of the sides of the skull are called the temples, to which the ears are attached. To the temple bones others are joined, which extend to the top or crown of the head, and attached to these are the bones which form the back part and base of the skull. The bones of the under part of the skull contain small holes, through which the nerves connected with the various organs of sensation communicate with the brain. Those parts of the front of the skull in which the eyeballs are placed are called orbits, above which there are firm ridges of bone, which, with the eyebrows attached to them, protect the eyes from injury. Beneath the orbits are placed the cheek bones, and between the cheek bones is the top or bony part of the nose, the lower or flexible part of which is composed of cartilage. The upper jaws adjoin the cheek bones, and the lower jaws have hinge joints near the holes of the ears, and by means of these hinges they are enabled to move in the various ways required. The teeth, which are intended for the mastication

or chewing of food, are fixed in the upper and lower jaws. Each tooth is composed of two parts -a bony substance, which forms the body of the tooth, and a thin covering called the enamel. That part which is out of the jaw is called the crown and neck of the tooth, while the fangs or roots are planted deep in the jaw. There is a small cavity in the body of the tooth, which contains the vessels and nerves which were employed in the first formation and afterwards in the nutrition of the tooth.

The teeth which are first seen are called milk teeth, or temporary teeth, from their being intended to continue only a few years. They are twenty in number, and are divided into three kinds namely, incisors, or cutting-teeth, of which there are eight; cuspidate, or dog-teeth, of which there are four; molares, or grinders, of which there are eight.

The permanent teeth are thirty-two in number, and are divided into three kinds: namely, incisors, of which there are eight; cuspidate, of which there are four; bi-cuspidate, of which there are eight; molares, of which there are twelve, four of the last being called dentes sapientiæ, or teeth of wisdom, from their not appearing till adult age.

The permanent teeth are more in number than the temporary, because of the disproportion between the size of the jaw in the young and adult. It was necessary when the jaw became

larger that not only a supply of larger teeth but also additional teeth should be given.

The Trunk.

The spine, which is in the form of an italic f, consists of twenty-four bones, each bending and making a joint with its fellow.

These are

At

called the twenty-four moveable vertebra. the lower part of the spine is placed a firm, thick, strong, bony structure called the haunches, or hips, and sacrum. These encircle a sort of

hollow space termed the pelvis or basin. The spine rests on the sacrum, which comprises five bones, and near the sacrum are four others called caudal bones. As these bones have no motion they are called the nine immoveable vertebra. In the centre of the bones of the spine there is a tube containing a substance called the spinal cord or marrow, which is a prolongation of the brain and resembles it a good deal in its nature and functions. The bones of the spine or vertebral column all yield, in a slight degree, and permit in its whole line that degree of flexibility which is necessary to the motions of the body. Between these bones there is an elastic gristly substance which permits them to approach and play a little in the actions of the body. Whenever a weight is upon the head this gristle yields, and the moment it is removed. the gristle regains its place, and the bones re

sume their position. The spine yields, recoils, and forms the most perfect spring, calculated to carry the head without jar or injury. The breadth of the basis on which the parts severally rest, and the closeness of the junction, give to the chain its firmness and stability; the number of parts and consequent frequency of joints its flexibility. This flexibility varies in different parts of the chain-is least in the back, where strength more than the power of bending is wanted; greater in the loins, which it was necessary should be more supple than the back; and greatest of all in the neck, for the free motion of the head. In order to afford a free passage for the descent of the spinal marrow, each of these bones is bored through in the middle, in such a manner as that when put together the hole in one bone falls into a line and corresponds with the holes in the two bones meeting it, so by that means the perforated pieces, when joined, form an entire, close, uninterrupted channel, at least while the spine is upright and at rest. But as a settled posture is inconsistent with its use, the bones or vertebræ are so locked in and confined as to prevent them from shifting one upon another as the body moves or twists, and thus they maintain their relative position nearly unaltered. For the marrowcanal, giving out in its course a supply of nerves to different parts of the body, notches are made on the upper and lower edge of each.