The =optic nerve= (N. opticus) is composed of fibers which are the
central processes of the ganglion cells of the retina. The fibers
converge within the eyeball to the optic papilla, where they are
collected into a round trunk, the optic nerve. The nerve thus formed
pierces the chorioid and sclera, emerges from the posterior part of the
eyeball, and passes backward and inward to the optic foramen. After
traversing the latter it decussates with its fellow of the opposite side
to form the optic chiasma or commissure. In the orbit the nerve is
slightly flexuous and is embedded in the fat behind the eyeball and
surrounded by the retractor oculi muscle. Its intraosseous part is an
inch or more (ca. 3 cm.) long. The =sheath= of the nerve is formed by
prolongations of the membranes of the brain, and includes continuations
of the subdural and subarachnoid spaces.
The greater part of the fibers of the optic nerve cross in the chiasma
to the tract of the opposite side. In the tract the fibers proceed to
(1) the internal geniculate body, (2) the posterior part of the
thalamus, and (3) the anterior quadrigeminal body (indirectly). The
fibers which go to the internal geniculate body appear to belong to
Gudden’s commissure and to be nonvisual in function. The visual
fibers, which come from the outer part of the retina of the same side
and the inner part of the retina of the opposite side, terminate about
cells in the anterior quadrigeminal body and the part of the thalamus
which corresponds to the pulvinar and external geniculate body of man.
From the cells of the former fibers pass to the nuclei of the motor
nerves of the eyeball, and complete the reflex arc. Fibers proceed
from the cells of the thalamus to the visual area of the cortex in the
occipital part of the hemisphere.