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THE NERVOUS SYSTEM. 281

and the external rectus. It arises superficially from the walls of the
interpeduncular space on the median surface of the cms cerebri, just
above the pons varolii. It extends forward and slightly outward to the
side of the posterior clinoid process, soon after passing which it enters
the superior lateral portion of the cavernous sinus, being invested by a
sheath from the dura mater. It runs through this portion of the sinus,
passes below the anterior clinoid process, and on to the proximal
extremity of the anterior lacerated foramen. Here it enters the orbit
by passing between the two heads of the external rectus muscle. As
it extends through the anterior lacerated foramen it divides into two
branches, superior and inferior.
The Superior Division of the Oculo-motor Nerve is the smaller of the
two. It passes inward over the optic nerve, and again divides into two
sets of branches, one being distributed to the superior rectus muscle,
while the other supplies the levator palpebrse superioris.
The Inferior Division of the Oculo-motor Nerve is the larger of the
two. It divides into three branches—the internal rectus, inferior
rectus, and inferior oblique.
The Internal Eectus Nerve passes beneath the optic nerve and supplies
the internal rectus muscle.
The Inferior Rectus Nerve supplies the inferior rectus muscle.
.
The Inferior Oblique Nerve is the longest of the three branches. It
passes forward between the inferior and external recti muscles to the
inferior and anterior portion of the orbit, and is mainly distributed to
the inferior oblique muscle. It also sends a few filaments to the inferior
rectus, and a short, thick communicating branch to the ophthalmic or
lenticular ganglion.

Trochlear Nerve.
The trochlear, fourth, or patheticu§ nerve (Fig. 141) is the smallest
and the most slender of all the cranial nerves, though it has the longest
encranial course. It presides over the motion of the superior oblique
or trochlear muscle of the eye. It arises superficially from a point just
below the corpora quadrigemina and near the valve of Vieussens. From
this point it passes outward over the superior peduncle of the cerebel-
lum, then forward, curves around the lateral margin of the crus cerebri,
and penetrates the dura mater below the tentorium cerebelli. Near
the posterior clinoid process it enters the cavernous sinus, extends
along its outer and upper wall, and passes through the proximal por-
tion of the anterior lacerated foramen into the orbit. It then passes
forward and inward* over the superior rectus and le\'ator palpebrse
superioris muscles, and is distributed to the upper surface of the
superior oblique.
Branches.—Recurrent branches of this nerve are given off as it ])asses
through the tentorium cerebelli. They are distributed to the tentorium,
some of them extending backward to the lateral sinuses. In the cav-
ernous sinus it gives off branches which communicate with the carotid
plexus of the sympathetic nerve, and occasionally with the ophthalmic
division of the fifth nerve. It sometimes sends a branch which anasto-