The medial check ligament is an extension from the fascia covering the medial rectus muscle and attaches immediately posterior to the posterior lacrimal crest of the lacrimal bone.
The lateral check ligament is an extension from the fascia covering the lateral rectus muscle and is attached to the orbital tubercle of the zygomatic bone.
Functionally, the positioning of these ligaments seems to restrict the medial and lateral rectus muscles, thus the names of the fascial specializations.
There are two groups of muscles within the orbit: extrinsic muscles of eyeball (extra-ocular muscles) involved in movements of the eyeball or raising upper eyelids, and intrinsic muscles within the eyeball, which control the shape of the lens and size of the pupil.
The extrinsic muscles include the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique.
The intrinsic muscles include the ciliary muscle, the sphincter pupillae, and the dilator pupillae.
Of the seven muscles in the extrinsic group of muscles, one raises the eyelids, whereas the other six move the eyeball itself (Table 8.8).
The movements of the eyeball, in three dimensions, (Fig. 8.93) are: elevation—moving the pupil superiorly, depression—moving the pupil inferiorly, abduction—moving the pupil laterally, adduction—moving the pupil medially, internal rotation (intorsion)—rotating the upper part of the pupil medially (or toward the nose), and external rotation (extorsion)—rotating the upper part of the pupil laterally (or toward the temple).
The axis of each orbit is directed slightly laterally from back to front, but each eyeball is directed anteriorly (Fig. 8.94). Therefore the pull of some muscles has multiple effects on the movement of the eyeball, whereas that of others has a single effect.
Levator palpebrae superioris raises the upper eyelid (Table 8.8). It is the most superior muscle in the orbit, originating from the roof, just anterior to the optic canal on the inferior surface of the lesser wing of the sphenoid (Fig. 8.95B). Its primary point of insertion is into the anterior surface of the superior tarsus, but a few fibers also attach to the skin of the upper eyelid and the superior conjunctival fornix.
Innervation is by the superior branch of the oculomotor nerve [III].
Contraction of the levator palpebrae superioris raises the upper eyelid.
A unique feature of the levator palpebrae superioris is that a collection of smooth muscle fibers passes from its inferior surface to the upper edge of the superior tarsus (see Fig. 8.77). This group of smooth muscle fibers (the superior tarsal muscle) help maintain eyelid elevation and are innervated by postganglionic sympathetic fibers from the superior cervical ganglion.
Loss of oculomotor nerve [III] function results in complete ptosis or drooping of the superior eyelid, whereas loss of sympathetic innervation to the superior tarsal muscle results in partial ptosis.
Four rectus muscles occupy medial, lateral, inferior, and superior positions as they pass from their origins posteriorly to their points of attachment on the anterior half of the eyeball (Fig. 8.95 and Table 8.8). They originate as a group from a common tendinous ring at the apex of the orbit and form a cone of muscles as they pass forward to their attachment on the eyeball.
The superior and inferior rectus muscles have complicated actions because the apex of the orbit, where the muscles originate, is medial to the central axis of the eyeball when looking directly forward:
The superior rectus originates from the superior part of the common tendinous ring above the optic canal.
The inferior rectus originates from the inferior part of the common tendinous ring below the optic canal (Fig. 8.96).