Separating the lateral wall of the orbit from the floor of the orbit is a longitudinal opening, the inferior orbital fissure (Fig. 8.88). Its borders are the greater wing of the sphenoid and the maxilla, palatine, and zygomatic bones. This long fissure allows communication between: the orbit and the pterygopalatine fossa posteriorly, the orbit and the infratemporal fossa in the middle, and the orbit and the temporal fossa posterolaterally.
Passing through the inferior orbital fissure are the maxillary nerve [V2] and its zygomatic branch, the infra-orbital vessels, and a vein communicating with the pterygoid plexus of veins.
Beginning posteriorly and crossing about two-thirds of the inferior orbital fissure, a groove (the infra-orbital groove) is encountered, which continues anteriorly across the floor of the orbit (Fig. 8.88). This groove connects with the infra-orbital canal that opens onto the face at the infra-orbital foramen.
The infra-orbital nerve, part of the maxillary nerve [V2], and vessels pass through this structure as they exit onto the face.
Associated with the medial wall of the bony orbit are several smaller openings (Fig. 8.88).
The anterior and posterior ethmoidal foramina are at the junction between the superior and medial walls. These openings provide exits from the orbit into the ethmoid bone for the anterior and posterior ethmoidal nerves and vessels.
Completing the openings on the medial wall is a canal in the lower part of the wall anteriorly. Clearly visible is the depression for the lacrimal sac formed by the lacrimal bone and the frontal process of the maxilla. This depression is continuous with the nasolacrimal canal, which leads to the inferior nasal meatus. Contained within the nasolacrimal canal is the nasolacrimal duct, a part of the lacrimal apparatus.
The periosteum lining the bones that form the orbit is the periorbita (Fig. 8.90A). It is continuous at the margins of the orbit with the periosteum on the outer surface of the skull and sends extensions into the upper and lower eyelids (the orbital septa).
At the various openings where the orbit communicates with the cranial cavity the periorbita is continuous with the periosteal layer of dura mater. In the posterior part of the orbit, the periorbita thickens around the optic canal and the central part of the superior orbital fissure. This is the point of origin of the four rectus muscles and is the common tendinous ring.
Fascial sheath of the eyeball
The fascial sheath of the eyeball (bulbar sheath) is a layer of fascia that encloses a major part of the eyeball (Figs. 8.91 and 8.92):
Posteriorly, it is firmly attached to the sclera (the white part of the eyeball) around the point of entrance of the optic nerve into the eyeball.
Anteriorly, it is firmly attached to the sclera near the edge of the cornea (the clear part of the eyeball).
Additionally, as the muscles approach the eyeball, the investing fascia surrounding each muscle blends with the fascial sheath of the eyeball as the muscles pass through and continue to their point of attachment.
A specialized lower part of the fascial sheath of the eyeball is the suspensory ligament (Figs. 8.91 and 8.92), which supports the eyeball. This “sling-like” structure is made up of the fascial sheath of the eyeball and contributions from the two inferior ocular muscles and the medial and lateral ocular muscles.
Check ligaments of the medial and lateral
Other fascial specializations in the orbit are the check ligaments (Fig. 8.92). These are expansions of the investing fascia covering the medial and lateral rectus muscles, which attach to the medial and lateral walls of the bony orbit: