Each orbit is a pyramidal-shaped space above the upper jaw. The upper surface of the maxilla makes the greater part of the floor, but the malar comes into the outer par,, the hamular process of the lachrymal may be in the floor internally, and the orbital process of the palate is visible in the extreme back part of the floor at its junction with the inner wall (see Fig. 185).
The floor is separated from the outer or posterior wall by the spheno-maxillary fissure, except at its extreme outer end, where the malar is continued from the floor to the outer wall. The remaining and greater part of the outer wall is made by the orbital surface of the great wing. The outer wall is separated from the roof by the sphenoidal fissure in its back part, but outside this the great wing and the malar articulate with the frontal.
The roof is almost entirely formed by the orbital plate of the frontal, but the small wing of the sphenoid constitutes its most posterior portion. The inner wall is formed by the body and turbinate of the sphenoid, in front of this the ethmoid, and in front of this the lachrymal and the nasal process of the maxilla ; it has the anterior and posterior ethmoidal canals between it and the roof, above the os planum, and the nasal duct opens between it and the floor.
The axis of the orbit is directed backwards and inwards and slightly upwards, from the centre of the anterior opening to the optic foramen : the optic nerve lies practically in the line of the axis.
The margins of the anterior opening are made by several bones : above, the frontal, internally the nasal process of the maxilla, the maxilla and nuxlar below, and the malar externally. Very rarely the hamular process of the lachrymal may be seen to reach the margin of the orbit. The margins are sharp except at the inner and upper angle, internal to the supraorbital foramen.
Owing to the obliquity of the axis the inner walls of the orbits are in nearly parallel planes, whereas the outer walls are in planes practically at right angles to each other. The depth of the fossa is about 2 inches.
The lachrymal sac lies in the lachrymal groove near the margin of the opening ; the Tensor tarsi arises from the crest behind the groove and passes forward to the outer side of the sac to join the Orbicularis palpebrarum, and the tendo oculi arises from the nasal process in front of the groove and passes out across the middle of the sac.