The orbital cavity is theprotec-tive bony socket for the globe together with the optic nerve, ocular muscles,nerves, blood vessels, and lacrimal gland. These structures are surrounded by orbital fatty tissue. The orbital cavity is shaped like a funnel that opens anteri-orly and inferiorly. The six ocular muscles originate at the apex of the funnel around the optic nerve and insert into the globe. The globe moves within the orbital cavity as in a joint socket.
This consists of seven bones (Fig. 15.1):
The bony rim of the orbital cavity forms a strong ring. Its other bony surfaces include very thin plates of bone (see adjacent structures).
The close proximity of the orbital cavity to adjacentstructures is clinically significant. The maxillary sinus inferior to the orbital cavity is separated from it by a plate of bone 0.5 mm thick. The ethmoidalair cells located medial and posterior to the orbital cavity are separatedfrom it by a plate of bone only 0.3 mm thick or by periosteum alone. The fol-lowing other structures are also located immediately adjacent to the orbital cavity.
❖ Sphenoidal sinus.
❖ Middle cranial fossa.
❖ Region of the optic chiasm.
❖ Pituitary gland.
❖ Cavernous sinus.
Superior adjacent structures include the anterior cranial fossa and the fron-tal sinus. Table 15.1 lists the various bony openings into the orbital cavity and the anatomic structures that pass through them.
Because of this anatomic sit-uation, the orbital cavity is frequently affected by disorders of adjacent struc-tures. For example, inflammations of the paranasal sinuses can result in orbi-tal cellulitis.
The walls of the orbital cavity are lined with periosteum, which is also referred to as periorbita. Its anterior boundary is formed by the orbital septa extending from the orbital rim to the superior and inferior tarsal plates, the lateral and medial palpebral ligaments, and the eyelids.
The orbital cavity is supplied by theophthalmic artery, abranch of the internal carotid artery. The ophthalmic artery communicates with the angular artery, a branch of the external carotid artery, via the supraorbital and supratrochlear arteries.
Stenosis of the internal carotid artery can result in reversed blood flow through the supraorbital and supratrochlear arteries. This can be dem-onstrated by Doppler ultrasound studies.
The orbital cavity drains throughthe inferior ophthalmic vein into the pterygoid plexus, through the superiorophthalmic vein into the cavernous sinus, and through the angular vein intothe facial veins.