SKULL, the skeleton of the head, composed of 22 bones, 8 of which form the skeleton of the cranium, 14 that of the face. Ex cept the lower jaw, which is mov able, the bones are united by im movable joints. In the following article the skull is considered as a whole, and for this purpose a normal European skull is stud ied from in front, from above, from the side, from behind and from below. Afterwards the in terior is considered by means of sections.
The Skull from in Front (see fig. i).—The forehead re gion is formed by the frontal bone, the two halves of which usually unite in the second year. The lower limit of the forehead is formed by the upper mar gin of the orbit on each side, and by the articulation between the frontal and nasal bones near the mid line. At the junc tion of the inner and middle third of each supra-orbital mar gin is the supra-orbital notch for the nerve of that name. Above each supra-orbital margin is an elevation, better marked in adult males, called the supra-ciliary ridge, while between these ridges in the middle line there is a slight prominence, the glabella. Below the forehead the two nasal bones form the skeleton of the upper part of the nose; they articulate with one another in the mid line, but laterally they are joined by a suture to the nasal processes of the maxillae which run up to articulate with the frontal at the internal orbital process, thus forming the inner margin of the orbit.
Externally the malar bones articulate with the frontal at the external orbital process and form the lower and outer quadrant of the orbital margin.
The maxillae or upper jaws form the greater part of the skele ton of the face; they complete the lower and inner quadrant of the orbit, and below the nasal bones leave the anterior nasal aper ture between them, and project slightly at the middle of the lower border of this aperture to form the anterior nasal spine. About a quarter of an inch below the infra-orbital margin and just below the articulation with the malar the infra-orbital foramen, for the infra-orbital branch of the fifth nerve, is seen on each side. The
lower parts of the maxillae form the alveolar margin in which all the upper teeth are set. Laterally each maxilla is prolonged out into the zygomatic process, which supports the malar bone.
Below the maxillae the mandible or lower jaw is seen in per spective. The horizontal part or body is in two halves up to the second year, but after that complete bony union takes place, form ing the symphysis. Above the body of the mandible is an alveo lar margin containing the sockets of the lower teeth, while below, near the mid line, the bone projects forward to a variable extent and so forms the mental prominence, one of the special character istics of a human skull. Below the second bicuspid tooth on each side is the mental foramen for the exit of the mental branch of the fifth nerve.
The Orbit.—Each orbit is a pyramidal cavity, the base of the pyramid being in front and the apex behind, at the optic foramen, where the optic nerve and ophthalmic artery pass through. The four sides of the pyramid form the roof, floor, inner and outer walls of the orbit. The roof and floor are arched from side to side, the inner wall is antero-posterior and parallel with its fellow of the opposite orbit; the outer wall slopes backward and inward, the two opposite sides therefore converge as they run back. Seven bones enter into the composition of the orbit, viz. the frontal, sphenoid, maxilla, malar, lacrymal, ethnoid and palate. Between the roof and the outer wall, behind, is a slit in the sphenoidal fis • sure which transmits the third, fourth, first division of the fifth and sixth cranial nerves and the ophthalmic vein. Another slit (spheno-maxillary fissure) lies in the line of junction of the outer wall and floor, it leads into the spheno-maxillary and zygomatic fossae and transmits the second division of the fifth nerve and some veins.