The Orbital Fasciæ and Ocular Muscles (Figs. , , )

The straight muscles of the eyeball and the oblique muscles in part are inclosed in fibrous sheaths (Fasciæ musculares), formed by superficial and deep layers of fascia, which are united by intermuscular septa in the interstices between the muscles. The superficial fascia is thin; it blends in front with the fibrous layer of the eyelids and is attached behind around the optic foramen. The deep fascia consists anteriorly of two layers, one of which is continuous with the fibrous tissue of the lids, while the other is attached at the corneo-scleral junction.

The posterior part of the eyeball is covered by the bulbar fascia or capsule of Tenon (Fascia bulbi), so that between them a lymph space (Spatium interfasciale) is inclosed which communicates with the subdural space along the course of the optic nerve.

The levator palpebræ superioris muscle is a thin band about half an inch in width which lies above the rectus superior. It is narrow at its origin above and behind the ethmoidal foramen and ends by an expanded tendon in the upper lid. Its action is to raise the upper lid.

The recti oculi (Mm. recti bulbi) are four in number and are designated according to their positions as superior, inferior, internal, and external. They are all band-like, arise close together around the optic foramen, and diverge as they pass forward to the eyeball. On reaching the latter they end in thin tendons which are inserted into the sclera in front of the equator of the eyeball.

The retractor oculi (M. retractor bulbi) surrounds the optic nerve, and is incompletely divided into four parts which alternate with the recti. They arise around the optic nerve and are inserted into the sclera behind the recti.

The obliquus oculi superior (M. obliquus bulbi dorsalis s. superior) is the longest and narrowest of the ocular muscles. It arises near the ethmoidal foramen and passes forward internal to the rectus internus. Under the root of the supraorbital process it is reflected almost at a right angle around a cartilaginous pulley (trochlea), which is attached to the anterior part of the inner wall of the orbit, a bursa being interposed here. The muscle is then directed outward and somewhat forward, and ends in a thin tendon which passes between the rectus superior and the eyeball, and is inserted into the sclera between the superior and external recti, about half an inch behind the margin of the cornea.

The obliquus oculi inferior (M. obliquus bulbi ventralis s. inferior) is wide and much shorter than the recti. It arises from the inner wall of the orbit in the small depression (Fossa muscularis) behind the lacrimal fossa. It curves around the rectus inferior and is inserted into the sclera near and partly beneath the rectus externus.

Actions.—The superior and inferior recti rotate the eyeball about a transverse axis, moving the vertex of the cornea upward and downward respectively. Similarly the internal and external recti rotate the eyeball about a vertical axis, turning the vertex of the cornea inward and outward respectively. The oblique muscles rotate the eyeball about a longitudinal axis; the superior oblique raises the outer end of the pupil, while the inferior oblique lowers it. The retractor as a whole draws the eyeball backward, and its parts may separately reinforce the corresponding recti. Also the four recti acting together will retract the eyeball.

The actual movements of the eyeball are by no means so simple as might be inferred from the foregoing general statements. Practically all movements are produced by the coördinated actions of several muscles, involving combinations which are quite complex and difficult to analyze accurately. Further complication is caused by the fact that the recti are not inserted at equal distances from the equator and the axes of rotation of the oblique muscles do not correspond to the longitudinal axis of the eyeball.

Nerve-supply.—The oculomotor nerve supplies the foregoing muscles, with the exception of the rectus externus and obliquus superior, which are innervated by the abducens and trochlearis respectively.