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[217] 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.
[Illustration: