THB PERIPHER/IL NERVOUS SYSTEM. 3^9 



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. 



I. Cranial Nerves. Nervi cerebrales. — I. N. olfac- 

 TORIUS. — The fasciculi of the olfactory nerves arise from the 

 olfactory bulb (Fig. 144, a) 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. OPTICUS. — The second nerve or optic (Fig. 138, 

 //) 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. 



III. N. OCULOMOTORIUS. — 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,/). 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 

 ophthalmic nerve. From the ciliary ganglion proceed two- 



