250 H. W. NORRIS 
The optic nerve fibers enter the brain at the end of a slender 
hollow stalk (fig. 10). Cells of the ventral wall of this stalk 
extend out into the nerve trunk and form a central core that ex- 
tends through the nerve even to the entrance of the nerve into 
+ mtl. ext. 
Fig. 7 A cross-section through the anterior part of the eyeball and posterior 
edge of the postnares. The anterior end of the antorbital cartilage (ao) is shown. 
Section 160. X20. 
the eyeball. The fibers of the optic nerve run into the brain 
along the posterior wall of the hollow stalk. Externally to the 
brain the optic nerve extends anteriorly along the medial border 
of the trabecula (orbito-sphenoid cartilage). Emerging from the 
Fig. 8 A cross-section through the middle of the eyeball. Shows the antor- 
bital cartilage, its attachment to the skull, and the two antorbital muscles (rtao. 
and lvao.). The alveolar branch of the ramus mandibularis V (md. 4a) is seen 
passing over the jaw to join the alveolaris VII (alv. 1). Section 170. 20. 
Fig.9 <A cross-section through the eyeball, a little posterior to the preceding. 
Section 175. 20. : 
Fig. 10 A cross-section through the external origin of the optic nerve. All 
the eye-muscle nerves are shown. The trochlearis is passing out through a fora- 
men in the parietal bone. Section 290. 20. ; 
