The orbital cavities are large bony sockets that house the eyeballs with associated muscles, nerves, blood vessels and fat. Each orbit is pear-shaped with the optic nerve representing the stem. The medial walls of the orbit are almost parallel and border the nasal cavity anteriorly. The lateral walls, however, diverge at an angle of 45 degrees.
Seven bones make up the orbit, namely:
The bones comprising the lateral wall and roof tend to be thicker than the more fragile bones of the medial wall and floor. The latter are therefore more vulnerable to fracture with blunt trauma. A particularly paper-thin area of the medial wall is the lamina papyracea which can be a route of entry for infection into the orbit from the adjacent ethmoid sinus. The lacrimal fossa sits anteriorly in the medial wall and is continuous with the bony nasolacrimal canal which extends into the nose.
Main orbital openings
The optic canal at the orbital apex connects the orbital cavity to the middle cranial fossa. It transmits the optic nerve and ophthalmic artery, a branch of the internal carotid artery, which is the main arterial supply to the eye and orbital contents. The central retinal artery branches off the ophthalmic artery just after it enters the orbit, piercing the optic nerve just behind the globe to supply the retina. Blood leaves the eye via the central retinal vein, which travels out of the optic nerve next to the artery before draining into the superior ophthalmic vein.
Superior Orbital Fissure
The superior orbital fissure lies between the lesser and greater wings of the sphenoid. Passing through this fissure to enter the orbit are several important structures including the:
Oculomotor (3rd) nerve which supplies all of the extra-ocular muscles, except the superior oblique and lateral rectus, and also innervates the levator muscle in the upper eyelid. In addition the oculomotor nerve carries parasympathetic fibres, which synapse in the ciliary ganglion, to the ciliary muscle (involved with accommodation) and sphincter pupillae (causes pupil constriction).
Earlier in its course the oculomotor nerve, having emerged from the anterior aspect of the midbrain, travels parallel to the posterior communicating artery and is, therefore, vulnerable to damage if compressed by an aneurysm in this vessel.
Trochlear (4th) nerve, which is the most slender cranial nerve, and supplies the superior oblique muscle. It is the only cranial nerve to arise from the posterior surface of the midbrain and can be susceptible to damage with trauma.
Abducens (6th) nerve which supplies the lateral rectus muscle. It has a long course from the lower border of the pons, including travelling across the sharp border of the petrous part of the temporal bone. It can therefore be damaged in head injuries and prone to stretching should the brain stem be displaced in raised intracranial pressure (“false localising” 6th nerve palsy).
The superior ophthalmic vein passes back through the lateral part of the fissure to enter the cavernous sinus.
Surrounding the optic canal and the lower half of the superior orbital fissure is the common tendinous ring which is the origin of the four recti muscles.
Inferior Orbital Fissure
The inferior orbital fissure lies just below the superior orbital fissure, between the lateral wall and floor of the orbit. It transmits the infra-orbital nerve which can be harmed in blow-out fractures, resulting in parasthesia below the lower eyelid. The inferior ophthalmic vein also passes through its lower portion before entering the cavernous sinus.