THE PERIPHERAL NERVOUS SYSTEM. 369 
to the projections of the base of the skull and to the tentorium. 
It dips between the cerebral hemispheres and olfactory bulbs, 
forming the falx cerebri. It likewise dips between the cere- 
brum and the cerebellum, covering both surfaces of the bony 
tentorium. The dura mater does not pass into the fissures or 
sulci of the surface of the brain. It becomes continuous with 
the sheath of the cranial nerves as they leave the skull. It is 
continuous with the dura mater of the spinal cord. 
The arachnoid and pia mater are essentially similar to the 
same structures on the spinal cord. The pia mater dips into 
the fissures and sulci of the brain. 
II. THE PERIPHERAL NERVOUS SYSTEM. 
1. Cranial Nerves. Nervi cerebrales.—I. N. OLFAC- 
TORIUS.—The fasciculi of the olfactory nerves arise from the 
olfactory bulb (Fig. 144, @) and pass through the foramina of 
the cribriform plate, upon which the bulb lies, to be distributed 
to the olfactory mucous membrane of the nose. 
II. N. opricus.—The second nerve or optic (Fig. 138, 
fT) arises from the optic chiasma (Fig. 138, c), passes through 
the optic foramen, and extends in an S-shaped curve to the 
eyeball. Its course is craniodorsad. It pierces the sclerotic 
and choroid coats of the eye and spreads out into the retina. 
Ill. N. ocuLomMoTorius.—The third or oculomotor nerve 
arises (Fig. 138, ///) from the pedunculus cerebri and passes 
into the orbit through the orbital fissure. It passes between 
the lateral and superior recti, sends a large branch to the 
superior rectus, supplies the medial rectus and the retractor 
oculi, passes laterad of the optic nerve and supplies the inferior 
rectus, and sends a long branch to the inferior oblique (Fig. 
166, 7). At the point where the branch is given off to the 
inferior oblique muscle is the triangular reddish ciliary (or 
ophthalmic) ganglion, about two millimeters in diameter. It 
is directly attached to the inferior oblique branch of the third 
nerve and receives a delicate sensory filament from the 
