MOTORES OCULORUM PATHETICI. 395 



orbit ; the other, the one in question, which forms a sheath for the nerve, 

 and is lost in the sclerotic coat of the eyeball. After a short course within 

 the orbit the optic nerve pierces the sclerotic and choroid coats and ex- 

 pands into the nervous membrane of the eyeball, the retina. Near the 

 globe, the nerve is pierced by a small artery, the arteria centralis retince, 

 \vhich runs through the central axis of the nerve and reaches the internal 

 surface of the retina, to which it distributes branches. 



The commissure rests upon the processus olivaris of the sphenoid bone ; 

 it is bounded by the lamina cinerea of the corpus callosum in front, by the 

 substantia perforata on each side, and by the tuber cinereum behind. 

 Within the commissure the innermost fibres of the optic nerves cross each 

 other to pass to opposite eyes, while the outer fibres continue their course 

 uninterruptedly to the eye of the corresponding side. The neurilemma 

 of the commissure, as w r ell as that of the nerves, is formed by the pia 

 mater. 



THIRD PAIR. ^MOTORES OCULORUM. The motor oculi, a nerve of mo- 

 derate size, arises from the inner side of the crus cerebri, close to the pons 

 Varolii, and passes forward between the posterior cerebral and superior 

 cerebellar artery. It pierces the dura mater immediately in front of the 

 posterior clinoid process ; descends obliquely along the external wall of 

 the cavernous sinus ; and divides into two branches which enter the orbit 

 between the two heads of the external rectus muscle. The superior 

 branch ascends, and supplies the superior rectus and levator palpebra?. 

 The inferior sends a branch beneath the optic nerve to the internal rectus, 

 another to the inferior rectus, and a long branch to the inferior oblique 

 muscle. From the latter a short thick branch is given off to the ciliary 

 ganglion, forming its inferior root. 



The fibres of origin of this nerve may be traced into the grey substance 

 of the crus cerebri,* into the motor tract, f and as far as the superior fibres 

 of the crus cerebri. :f In the cavernous sinus it receives one or two fila- 

 ments from the cavernous plexus, and one from the ophthalmic nerve. 



FOURTH PAIR. PATHETICI (trochlearis). The fourth is the smallest 

 cerebral nerve ; it arises from the valve of Vieussens close to the the testis, 

 and winding around the crus cerebri to the extremity of the petrous portion 

 of the temporal bone, pierces the dura mater near the oval opening for the 



testis of one side. 9. The brachium anterius of the corpora qnadrigemina. a. The 

 brachium posterius. b. The origin of the fourth nerve, which may be seen descending 

 over the crus cerebri. c. The processus e cerebello ad testem, or superior peduncle of 

 the cerebellum, d. The band of fibres termed laqueus, the superior division of the fas- 

 ciculus olivaris crossing the superior peduncle of the cerebellum to enter the corpora 

 qtmdrigemina. Through the small triangular space in front of this band, crossed by 

 the fourth nerve, some of the fibres of the superior peduncle of the cerebellum may be 

 seen. e. The superior portion of the crus cerebri, termed tegmentum. /. Its inferior 

 portion, g. The third nerve, h. The pons Varolii. i. The crus cerebelli, or middle 

 peduncle of the cerebellum, k. The inferior peduncle derived from the corpus resti- 

 forme. The mass lying in the angular interval upon these is the superior peduncle. 

 /. The fifth nerve issuing from between the transverse fasciculi of the pons Varolii. ra. 

 The sixth nerve, n. The seventh nerve ; the inferior and smaller cord is the facial 

 m*rve, the superior and larger the auditory, o. The corpus olivare crossed inferiorly 

 by the superficial arciform fibres, p. The corpus pyramidale. q. The median poste 

 rior fasciculi of the medulla oblongata. r. The corpus restiforme. *. The spinal cord. 

 t. The fourth ventricle. 



* Mayo. -j- Solly. * Grainger. 



