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The Skeleton

bones, bone, fig, occipital, cavity, roof and called

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THE SKELETON.

The skeleton forms the framework of the body, and is composed of bones united by joints, in many cases plates or pads of gristle being interposed between the opposing bones to per mit of movement, the union being strengthened by fibrous bands, which bind the one bone to the other.

The skeleton is divided into head, trunk, and limbs, and is made up of more than 200 bones.

In Fig. 13o is pointing to the extreme side limit of the occipital bone. The sides of tho This includes the part inclosing the brain, called cranium, and the face, the former con taining eight bones, and the latter fourteen.

The eight bones of the cranium are the frontal, two parietal, the occipital, two temporal, the sphenoid, and the ethmoid.

The frontal (Figs. 12 and 13, A) forms the forehead and part of the vault of the skull, as well as part of the roof of the sockets for the eye-balls (called the orbits). In children the frontal bone is in two parts, by ad i vision passing down the middle. The parietal (a) bones form the great portion of the roof, and meet the occipital behind, which completes the roof, and is continued onwards to form a large part of the floor or base of the cranial cavity. The part of the occipital form ing the floor is pierced by a large opening called the foramen magnum, through which the spinal cord passes to reach the brain.

cavity are closed in by the temporal bonet (n), one on each side. These contain the organ of hearing. In the figure D' indicates a down. ward projection of the temporal bone, which is situated just behind the ear. It is called the mastoid process. The gap in the floor of the cranial cavity left by the occipital is filled up by the sphenoid, which thus stretches between the occipital behind it and the plates of the frontal occupying the roof of the eye-sockets in front of it. Part of the wing of the sphenoid is seen in the figures, and is marked c; but the main portion is at the base of the skull, and within, and is therefore not seen in the figures. The ethmoid is a very spongy bone, is not visible from the outside, and fills up the interval between the orbits. It forms the roof of the cavity of the nose, and is pierced by small openings for the passage of the nerves of smell. Fig. 13, e, shows the

side of the ethmoid forming part of the inner wall of the orbit ; the other side of the bone in in a similar position on the inner wall of the other orbit.

The fourteen bones of the face are—two upper jaw-bones (superior maxillary), two malar or cheek bones, two nasal, two palate, two lacrymal, two inferior turbinated bones in the nasal cavity, the vomer or ploughshare, and the lower jaw-bone (inferior maxillary). The upper jaw-bones (F) carry all the upper teeth, and form part of the floor of the orbit, the rest of which is completed by the cheek bones (a), which also send an arch backwards to join the temporal bone. These arches (zygo matic arches r, Fig. 13) are the prominent ridges which run out from below the outer angle of the eye to the front of the ear. The nasal bones (a) form the bridge of the nose, and at their upper end they come into contact with the lacrymals (11, Fig. 13), placed in the inner angle of the orbit, and grooved for a duct, along which the tears pass from the eye to the nose. The cavity of the nose is divided into two by the vomer Fig. 12), so called from its resemblance to a ploughshare, which forms a middle wall of partition between the two nostrils, while the inferior turbinated are scroll-like bones which project from the wall in the inside of the cavities (id and L, Fig. 12). The palate bones are behind those of the upper jaw, and with them form the bony part of the roof of the mouth. The lower jaw bone (a) is the largest of the face bones, and carries all the lower teeth. It is the only bone in the head which is movable, a hinge joint being formed between its strong prominences, projecting upwards (p, Fig. 13), and a hollow in the temporal bone under the ends of the zygomatic arch. All the other bones of the head are immovably connected with one an other, one bone presenting a ragged edge, like badly-formed teeth of a saw, the teeth fitting into corresponding notches in the edge of the other bone. These irregular lines of union are called sutures.

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