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BONES OF THE SKULL.

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prevents the temporal bone from being driven inwards, as it might have otherwise been, by a blow on the side of the head.

618. In the base or floor of the cavity of the cranium are seen a number of apertures, which serve for the passage of the blood-vessels that supply the brain, and of the nerves that issue from it. One of these apertures, much larger than the rest, and situated in the occipital bone, gives passage to the Spinal Cord; and on each side of this aperture there is a large bony projection from the under surface, termed the condyle, by which the skull rests on the vertebral column, and is enabled to move forwards or backwards upon it. The head is nearly balanced upon this pivot; nevertheless, the portion situated in front of the joint is more heavy than that which is situated behind it, and is consequently not altogether counterpoised by the latter. Hence the muscles which, arising from the back and being attached to the occipital bone, tend to draw the head backwards, and thus to keep it upright, are more numerous and powerful than those which are situated in front of the vertebral column, and which tend to draw the head downwards and forwards; and when the former are relaxed, as in a person sleeping upright, the head has a tendency to fall forwards upon the chest. In no other animal is this joint situated so far forwards as in Man. As we descend the scale, we find it nearer and nearer to the back of the skull; and consequently the whole weight of the head bears, not directly upon the spine, but upon the muscles and ligaments by which

it is attached to the vertebral column.

619. On each side of the base of the cranium, we observe a large rounded projection, termed the mastoid. To this projection (which we feel behind the lower part of the ear) is attached on either side a powerful muscle, the sterno-mastoid (23, fig. 227), which passes downwards and towards the central line; so that the two muscles nearly meet at the bottom of the neck, where they are attached to the upper edge of the breastbone. These muscles, acting together, serve to draw the head forwards; but either of them acting separately will turn it to one side or the other. In front of these two projections of the skull, we notice the opening ta of the external ear; which, like the different chambers of the internal ear, is excavated in a portion of the temporal bone which is termed petrous from its very dense and stony character.

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BONES OF THE FACE.

620. The face is formed by the union of fourteen bones; and presents five large cavities, which serve for the lodgment and protection of the organs of sight, smell, and taste. All the bones of the face, with the exception of the lower jaw, are completely immovable, and are firmly united to each other and to the bones of the cranium (§ 617). The two principal are the superior maxillary (m s, fig. 220), which form nearly the whole of the upper jaw, and are connected with the frontal bone in such a manner as to contribute to the formation of the orbital cavities in which the eye is lodged, and of the nasal cavities which form the interior of the nose; they also constitute the front of the roof of the mouth; on the sides of the face, they articulate with the malar or cheek-bones j; whilst they are united behind with the palate-bones which form the back part of the roof of the mouth, and which in their turn are united to the sphenoid.

621. The orbits, as we have already seen (§ 538), are two deep cavities, of a conical form,—the base of the cone being directed forwards, and its apex or point backwards; the roof of these cavities is formed by a portion of the frontal bone, and their floor chiefly by the superior maxillary. Their inside wall is formed by the ethmoid bone, and by the small bone termed the lachrymal, in which is the canal for the passage of the tears into the nose (§ 541); and the outside wall is formed partly by the cheek-bone and partly by the sphenoid, the latter also bounding the cavity at its deepest part, and containing the apertures which serve for the passage of the optic and other nerves that enter the orbit from the cranium. In the roof of the orbit, on its outer side, there is a broad shallow pit or depression, in which the lachrymal gland is lodged.

622. The greater part of the nose is formed by cartilages; so that, in the bony skull, the anterior opening of the nasal cavity (na, fig. 220) is very large; and the bony portion of the nose, formed by the two small bones (n) termed nasal, projects but slightly. The nasal cavity, divided in the middle by a vertical partition into two fosse or excavations, is very extensive (§ 506); at the upper part it is hollowed-out into the ethmoid bone, the whole interior of which is made-up of large cells; its floor is formed by the arch of the palate, which separates it from the mouth; behind it extends as far

BONES OF THE FACE.

467

as the back of the mouth, and communicates with the pharynx by two apertures termed the posterior nares (fig. 200, c). The partition between the fosse is formed at the upper part by a plate that projects downwards from the ethmoid bone, and at the lower by a distinct bone called the vomer (or ploughshare) from its peculiar form; to the front edge of this last is attached a cartilage, which continues the partition forwards into the soft projecting portion of the nose. It is through the thin horizontal plate of the ethmoid bone, which separates the nasal cavity from that of the skull, that the olfactory nerves make their way out from the former into the latter: they descend in numerous branches, for the passage of which through the roof of the nose this plate is perforated by a number of small apertures, which give it a sieve-like aspect; whence it is called the cribriform plate of the

ethmoid.

623. It is in the superior maxillary bone that all the teeth of the upper jaw are implanted in Man; but in the embryo this bone is composed of several pieces; and one of these pieces, termed the intermaxillary bone (im, fig. 221), remains

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oc, occipital bone; t, temporal; f, frontal; n, nasal; m, superior maxillary; im, intermaxillary; mi, inferior maxillary; o, orbit; i, incisor teeth; c, canines; mo,

molars.

permanently separate in most of the lower animals. The lower jaw of adult Man, also, is composed but of a single piece; though this is divided in the infant on the central line, and the two halves remain separate in many of the lower animals. This bone has a general resemblance in form to a horse-shoe with its extremities turned up considerably; it is. 1 From the Latin, cribrum, a sieve.

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BONES AND MUSCLES OF THE FACE.

articulated with the temporal bones by a condyle or projecting head with which each of these extremities is furnished; and this head is received into what is called the glenoid1 cavity on the under side of the temporal bone. In front of the condyle is another projection, or process, termed the coronoid (a, fig. 92), which serves for the attachment of one of the principal muscles that raise the jaw. These muscles are all attached near the angle of the jaw (or the point at which it bends upwards), and they consequently act at a small distance from its fulcrum, whilst the resistance is applied at the furthest point (§ 180). We are continually reminded of the loss of mechanical power which results from this, by our inability to exercise the same force with our front teeth that we can employ with the back. Thus, when we wish to crack a nut, or to crush any hard substance between the teeth,, we almost instinctively carry it to the back of the jaws, so as to place it nearer the joint, where it may receive more of the power of the muscle.

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Fig. 222.

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624. The general arrangement of the chief muscles of the face is seen in fig. 222. The largest is the temporal muscle, t, the fibres of which arise from an extensive surface of the parietal and temporal bones, and then converge or approach each other, passing under the bony arch or zygoma, z (which is partly formed by a process from the temporal bone, and partly by the malar or cheek bone), to be attached to the a coronoid process of the lower jaw. This muscle is of extraordinary power in those beasts of prey which lift and drag heavy carcases in their jaws; and in those which (like the Hyæna) obtain their support by crushing the bones that others have left. It is assisted by the masseter muscle m, which passes from the zygomatic arch and cheek-bone to the angle of the lower jaw, and also by other muscles. Besides these, the figure shows. the ring-like muscle or sphincter o, which surrounds the opening of the eye, and serves by its contraction to close the lids; and also the similar muscle b b, which surrounds the

1 The term condyle is applied to most of the projecting surfaces of articulation, in different parts of the body; and the term glenoid to the cavities into which these are received.

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