Anatomy of the Eye
Daniel S. Casper
ORBITAL AND OCULAR ANATOMY
The eyes, or globes, are located in bilateral orbital cavities in the anterior, superior skull. These pear-shaped cavities have large anterior openings to permit vision and small apertures posteriorly for communication with the middle cranial fossae. The two bony orbits share three bones equally: the frontal, sphenoid, and ethmoid; four other paired bones—the maxillae, zygomas, lacrimal, and palatines—are duplicated and present individually on each side (Figure 24.1).
The boxy ethmoid sits behind the root of the nose, separating the orbits. Its thin, fragile lateral walls make up most of the medial orbital walls. The centrally located sphenoid bone traverses the skull and can be seen externally in each infratemporal fossa. It contains the sella turcica, which houses the pituitary gland. The sphenoid wings make up much of the lateral orbital walls, which are the interface between orbit and middle cranial fossa.
The orbits are arbitrarily divided into four walls that have no sharp boundaries. The roof is made up mostly of frontal bone, the medial walls are made of the lateral portion of the ethmoid; the floor is largely maxillary bone and overlies the maxillary sinus, and the lateral wall is mostly sphenoid sinus. The floor and medial walls are the thinnest and fracture more commonly with trauma. The lateral wall, recessed about a centimeter from the orbital opening, affords enhanced peripheral vision but also renders the lateral globe more susceptible to trauma owing to its greater exposure.
The posteriorly located apex houses the two major orbital portals to and from medial fossa. The optic canal carries the optic nerve, most of the orbital arterial supply via the ophthalmic artery, and sympathetic autonomic nerve fibers. Temporal to the optic canal is the superior orbital fissure, which carries all other neurovascular structures of importance, including cranial nerves III (oculomotor), IV (trochlear), V (the first trigeminal division, ophthalmic), and VI (abducens), as well as the superior ophthalmic vein and parasympathetic fibers.
Periorbital sinuses surround the orbits. The ethmoid sinus is located between the orbits; the sphenoid, posterior to the orbits; and the maxillary sinuses lie beneath the orbital floors. The paired frontal sinuses are located above the brows, medially, within the frontal bone (Figure 24.2).
The orbits are effectively enclosed spaces, with small posterior apical conveying structures to and from the middle cranial fossae, and anterior apertures (the palpebral fissures), from which the anterior globe protrudes slightly. A space-occupying lesion, such as an abscess, hemorrhage, or tumor located within the orbit will displace orbital structures, and the only outlet to accommodate such an increase in volume is the anterior opening. As orbital tissues are displaced, the eye is forced anteriorly, resulting in a bulging appearance known as proptosis, or exophthalmos (Figure 24.3).
Six extraocular muscles control movement and the position of each eye (Figure 24.4). The lateral and medial recti control horizontal movements; the superior and inferior recti primarily control vertical action; the superior and inferior oblique muscles bring about oblique (ie, nonvertical or horizontal) movements.
Horizontal muscles move the eye directly along the horizontal plane, left or right. The vertical recti, however, do not attach to the globe in the visual, anatomical plane, because the orbits deviate outward slightly from the medially located apices. Because of this off-axis alignment, muscle contraction does not simply elevate or depress the eye in the vertical axis; instead, there is a lateral component in addition to the main vertical direction. The oblique muscles similarly attach to the globes of the primary axis. Thus, the vertical recti and obliques must work together to move the eye directly up or down.
The superior oblique, unlike the other extraocular muscles, does not attach directly to the globe but instead travels through a fascial sling, the trochlea, in the superior medial orbit; the tendonous terminal portion reverses direction to attach to the posterior, superior globe. The inferior oblique, which originates from the orbital floor, follows the same course as the superior oblique tendon and attaches to the posterior, inferior globe. Oblique muscle action is counterintuitive: the superior deviates the eye downward and medially, and the inferior deviates the eye upward and medially. The consequence of this anatomic configuration is that vertical, upward (90°) eye movement requires the combined action of the superior rectus and inferior oblique and that the inferior rectus and superior oblique move the eye in a straight downward direction.
Sensory visual data processed by the retina is transmitted to the brain via cranial nerve II, the optic nerve. This nerve can be visualized with ophthalmoscopy at its origin as the optic disk in the posterior pole of the eye. After exiting the eye, it follows a sinuous course (which allows for free movement of the eye within the orbit) toward the apex of the orbit, where it enters the medially located optic canal and is conveyed into the middle cranial fossa, anterior to the pituitary gland. The right and left optic nerves comingle to form the optic chiasm. Postchiasmal optic tracts, comprised of combined fibers from both eyes, carry information derived from the contralateral visual hemifields to the lateral geniculate nuclei in the thalamus. From there, optic radiations proceed posteriorly to the occipital lobes, where visual perception occurs (Figure 24.5).