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THE NERVOUS SYSTEM. 279

The straight set form the outer part of the optic tract. They pass
forward, auJ help to ibrm the optic nerve of the same side.
The iutergeniculate set (interior commissure of Gudden) form the
inner part of the optic tract. They cross from one side to the other,
forming the ])osterior margin of the chiasm, and luiite the fibres which
spring from the geniculate bodies. Many anatomists describe a set of
fibres which pass from one side to the other across the anterior margin
of the chiasm and form an inter-retinal set. The existence of this set of
fibres is still a matter of doubt, though Stilling has recently claimed
to have found them.
The optic nerve is a rounded cord which commences at the anterior
lateral angle of the optic chiasm. It extends outward and forward, and
Fig. 140.

Diagram of the Optic Nerves and Tracts in IVraii : 1, left eyeball ; 2, right eyeball ; 3, 3, corpora genic-
ulata interna ; 4, 4, corpora geniculata externa ; 5, tubercula quadrigemina ; 6, 6, ceutres of vision
in the cerebral hemispheres.
passes from the brain-case through the optic foramen in the sphenoid
bone, accompanied by the ophthalmic artery, which runs along its outer
and lower side. Before entering the optic foramen it is invested by a
slender sheath from the arachnoid membrane of the brain, but as it
passes into the foramen it is strongly enveloped by a prolongation
from the dura mater. Upon reaching the orbit this covering divides
into two, the outer blending with the periosteum, while the inner con-
tinues to invest the nerve until it pierces the sclerotic coat of the eye.
When the nerve enters the orbit it passes outward and downward
between the origins of the recti muscles to the posterior aspect of the
eyeball, being surrounded by the adipose tissue of the orbit. It enters