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AREOLAR TISSUE, TENDONS, AND MUSCLES. 177

optic foramina. Fibres are giv^en off from the two heads of the muscle,
forming a tendinous arch over the foramen, through which pass the
third, the nasal branch of the fifth, and the sixth nerves, also the
ophthalmic vein.
The Superior Oblique, or Troehlearis, is a narrow elongated muscle
situated at the upper and inner part of the orbit, internal to the levator
palpebrse. It arises close to and in front of the inner margin of the
optic foramen. It extends forward to the upper and inner angle of the
orbit, where it becomes tendinous as it passes through a tibro-cartilagi-
nous ring or pulley (trochlea) attached to the trochlear fossa or process,
near the internal angular process of the frontal bone. The contiguous
surface of the tendon and ring is lined by a delicate synovial mem})rane
enclosed in a thin fibrous sheath. After the tendon passes through the
ring it resumes its fleshy appearance; it is deflected backward, outward,
and downward, and passes between the eye and the superior rectus, to
be inserted into the sclerotic coat, a little beyond the outer margin of
that muscle and midway between the cornea and the entrance of the
optic nerve.
The Inferior Oblique is a thin, narrow muscle situated near the ante-
rior margin of the orbit and close to the outside of the orifice of the
lachrymal duct. It arises from a slight depression in the orbital plate
of the superior maxillary bone near the lachrymal canal, from which it
passes outward, backward, and upward between the inferior rectus and
the floor of the orbit, and betMcen the external rectus and the eyeball,
terminating in a tendinous expansion which is inserted into the sclerotic
coat between the external and superior recti muscles near to the insertion
of the superior oblique.
Actions of the Orbital Muscles.— The Levator Palpebrce Supe-
rioris is the elevator of the upper eyelid, being antagonized by the upper
palpebral part of the orbicularis muscle, which is the closer of the eye.
The eyeball is so suspended within the orbit that it is easily moved
upon a fixed axis, but does not apparently change its position as a whole,
nor do the actions of the muscles make any distinct alteration in its
form. The fixed axis upon which the eye moves is nearly in the centre
of the curvature of the posterior wall, and a little back of the middle
of the antero-posterior axis of the eyeball.
The movement of the eye is best classified in four actions: (a) lateral
movement (in and out) : the inward motion is caused by the action of
the internal rectus, the outward by the action of the external rectus;
(6) perpendicular movement (up and down), the upward motion being
caused by the superior rectus, and the downward by the inferior rectus
muscles; (c) rotary movement, caused by the oblique muscles: the
superior oblique rotates the eye inward, and at the same time turns it
downward ; the inferior oblique turns it outward and upward. The
rotary movement of the eyeball is required when looking at an object
with the head inclined to either side, in order that the vision may
fall equally upon the retina of each eye. (d) Is a movement in which
two or more muscles act together ; for example, if the external and
superior rectus muscles are acting with equal power, the eyeball will be
directed in a line between the insertions of these muscles. It is by this
Vol. I.— 12