THE CRANIAL NERVES—THE OPTIC NERVE 793 
the cranial cavity through the foramen magnum, and divides at the interpeduncular 
fossa into the two posterior cerebral arteries. These concur with branches of the 
internal carotid arteries in forming the remarkable arterial circle on the base of the 
cerebrum. From these emanate basal arteries which supply in general the brain 
stem and basal ganglia, and cortical arteries which in general run superficially 
and supply the cortical substance and medullary white substance. 
The veins enter the sinuses of the dura mater, and in the main are not satel- 
lites of the arteries. The sinuses communicate with the ophthalmic, dorsal and 
ventral cerebral, and spinal veins. 
THE CRANIAL NERVES 
The cranial or cerebral nerves (Nn. cerebrales) comprise twelve pairs which 
are designated from before backward numerically and by name. Their number, 
names, and functional characters are given in the subjoined table: 
. 1. Olfactory. .....................Sensory (Smell) 
ESS OPtiCe Sts Saeed Nose eels oe Sensory (Sight) 
HUE ‘Oculomotor. ......--23: ee voto 
AVA eErochlear sy mies oe Does = Motor 
Weeinreminal’ a) ote... Maxed: 
VI. Abducent........ BaP Saat otcte es OLOR 
AVA cial Soya occ oon aaa stevens Mixed 
WHEE COUSUICI setae ee ete oo ...Sensory (Hearing and Equilibration) 
IX. Glosso-pharyngeal..............Mixed 
NORV ACTISt ee tM RA re Mixed 
Xf. Spinal accessory. ..-............Motor 
XD Eypoglossal So 25 oon... ses oe NLOUOL 
THE OLFACTORY NERVE 
The olfactory nerve (N. olfactorius) is peculiar in that its fibers are not aggre- 
gated to form a trunk, but are connected in small bundles with the olfactory bulb. 
They are non-medullated, and are the central processes of the olfactory cells which 
are situated in the olfactory region of the mucous membrane of the nasal cavity. 
This region is distinguished by its brown color and comprises the posterior part of 
the lateral mass of the ethmoid, a small adjacent area of the dorsal turbinate, and 
the corresponding surface of the septum nasi. The nerve-bundles are enclosed in 
sheaths derived from the membranes of the brain and pass through the foramina of 
the cribriform plate to join the convex surface of the olfactory bulb. Some 
fibers come from the vomero-nasal organ. 
THE OPTIC NERVE 
The optic nerve (N. opticus) is composed of fibers which are the central proc- 
esses 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 medially to the optic foramen. After 
traversing the latter it decussates with its fellow of the opposite side to form the 
optic chiasma. 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 intraos- 
seous part is an inch or more (ca.3cm.) 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 oppo- 
site side. In the tract the fibers proceed to (1) the medial geniculate body, (2) the posterior 
