492 NEUROLOGY. 



into two portions, one of which forms the periosteum of the orbit, 

 and the other a sheath for the nerve. After piercing the sclerotic 

 and choroid coats of the eyeball, the nerve expands into the 

 nervous retina. A little behind the eyeball, the nerve is pierced 

 to its centre for the passage of a small artery, the arteria centralis 

 retinae, which supplies the retina. 



In the optic commissure a peculiar decussation of nerve fibres 

 takes place ; while the outer fibres of the optic tract continue 

 their course direct to the eye of the same side, the innermost 

 pass to that of the opposite side ; at the same time, some of the 

 fibres cross from one tract to the other, and have no connection 

 with the eyes ; while others, it is said, proceed from eye to eye, 

 without having communication with the brain, 



THIRD PAIR — -MOTORES OCULORUM. 



This, the common motor nerve of the eye, has its apparent 

 origin on the inner side of the crus cerebri, close to the pons 

 Varolii, its course lying between the posterior cerebral and anterior 

 cerebellar arteries ; its deep origin may be traced to the locus 

 niger, pons Varolii, and corpora quadrigemina. It makes its exit 

 from the cranial cavity through the foramen lacerum orbitale, into 

 the orbital fossa, where it divides into several branches, supplying 

 the superior, internal, and inferior recti, inferior oblique, and 

 levator palpebrse muscles, and also branches to the lenticular 

 ganglion. 



FOURTH PAIR PATHETIC NERVES. 



This is a motor nerve, and the smallest of the cranial nerves ; 

 it has its superficial origin by the side of the pons Varolii, but its 

 deep fibres may be traced to the valve of Vieussens and the cor- 

 pora quadrigemina. Winding round the crus cerebri, this nerve 

 passes through the outer wall of the cavernous sinus, leaves the 

 cranium through the foramen patheticum, and is distributed to 

 the superior oblique muscle of the eye. It communicates with 

 the sympathetic, near the cavernous sinus. 



FIFTH PAIR TRIFACIAL NERVES. 



Called also the trigeminal, this is the largest of the cranial nerves, 

 and, being a nerve of common sensation, of motion, and indirectly, 

 perhaps, of .special sense, belongs to the class of mixed nerves. 



