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280 ANATOMY.

the eyeball about o'lie-tenth of an inch to the inner side of its centre,
passes through the sclerotic and choroid coat's, and terminates by
expanding into the retina. Within the orbit it is surrounded by adi-
pose tissue, ciliary vessels, and nerves, the central retinal artery enter-
ing the nerve about one-fourth of an inch from where it passes into the
sclerotic coat.
Variation.s.—The optic tracts occasionally pass through the optic
foramina without decussation. When this is the case the chiasm is
entirely absent.

OcuLO-MOTOR Nerve.
The oculo-motor or third nerve (Fig. 142) is a large, round, firm
cord which presides over the movements of the eye. It is the most
Fig. 141.

(Jleciirrent Filament
Meter Jtooi to Dura-Mater
t
Sensorv -flfl
Nerves of the Orbit, seen from above.
anterior motor nerve of the cerebro-spinal axis, and su])plies all the
nniscles of the orbit, including the spliincter muscles of the iris and the
ciliary muscle of the eyeball, with the exception of the superior oblique