The eyeball rests in its socket, which is hollowed out of the soft parts contained in the bony orbit. It is covered in front by the lids, which, as they slide over the eye, are lubricated by the tears. These are secreted by the lachrymal gland at the upper outer portion of the orbit, flow over the eye, and are drained off by the lachrymal canals and sac to empty into the nose through the lachrymonasal duct.

The Orbits

The orbits are large four-sided cavities, pyramidal in shape. The orbit in an adult male is about 4 cm. in diameter from side to side, and 3.5 cm. from above downward. The depth is 4.5 cm. It is thus seen that the orbit is wider than it is high. On receding into the orbit from its bony edge, the roof arches upward toward the brain to receive the lachrymal gland, thus making the up-and-down diameter slightly longer than the transverse.

The rim of the orbit is very strong and not readily broken by injuries. It is formed by the frontal bone above, the malar bone to the outside, the malar and superior maxillary below, and the superior maxillary and frontal to the inside. The inner (medial) walls of the two orbits are parallel, running distinctly anteroposte-riorly. The outer (lateral) walls diverge at an angle of about 45° from the inner ones.

The outer or lateral edge of the orbit is nearly or quite a centimetre and a half posterior to the inner or medial edge. This fact, together with the divergence of the outer wall, is the reason that, in enucleation of the eye, it is always tilted toward the nose, and the scissors introduced and the nerve cut from the outer side.

The outer wall of the orbital cavity is formed mainly by the broad flat surface of the greater wing of the sphenoid bone, and is thick and strong. The other three walls, on the contrary, are thin and weak. The thin orbital plate of the frontal bone above is frequently fractured in puncture wounds by foreign bodies, and the frontal lobe of the brain injured. Two such instances have come under the writer's care;

Fig. 86.   The bony orbit.

Fig. 86. - The bony orbit.

in the first case, an iron hook had penetrated and caused death from cerebritis; in the second, the wound was caused by a carriage pole. The patient recovered, notwithstanding a considerable loss of brain tissue.

Fig. 87.   Transverse section of the orbital and nasal cavities viewed from above.

Fig. 87. - Transverse section of the orbital and nasal cavities viewed from above.

To the medial side of the inner wall are the ethmoid cells, covered by the thin lachrymal bone and the os planum of the ethmoid. They are readily perforated by suppuration from within those cavities. The floor is chiefly formed by the thin orbital plate of the superior maxilla. In operations involving the floor of the orbit, care is necessary to avoid breaking through into the maxillary sinus (antrum) beneath.

At the edge of the junction of the outer and lower walls lies the inferior orbital (sphenomaxillary) fissure. It runs forward to within 1.5 cm. of the edge of the orbit and extends back to the apex of the orbit, where it unites with the superior orbital (sphenoidal) fissure, which lies between the roof and outer wall and extends forward about one-third of the distance to the edge of the orbit. The optic foramen enters the apex of the orbit at its upper and inner portion.

At the lower inner edge of the orbit is the lachrymal groove for the lachrymo-nasal duct, leading from the eye to the inferior meatus of the nose. At the junction of the middle and inner thirds of the upper edge is the supra-orbital notch. This can be felt through the skin. It transmits the supra-orbital artery and nerve. If a complete foramen is present instead of a notch, its location cannot be so readily determined.