CEREBELLUM, PONS, AND MEDULLA. 



243 



These two latter muscles belong to the type of plain muscle, 

 and the fibers of the third nerve which innervate them terminate 

 in the ciliary ganglion, whence the path is continued by sym- 

 pathetic nerve fibers (postganglionic fibers) to the muscles. In 

 the interior of the brain the fibers of the third nerve arise from a 

 conspicuous nucleus or collection of nuclei situated in the cen- 

 tral gray matter of the midbrain at the level of the superior col- 

 liculus. The fibers for the ciliary muscle and sphincter pupillse 

 arise more anteriorly than those for the extrinsic muscles. His- 

 tologically three parts at least may be distinguished, as shown in 

 Fig. 107, namely, the lateral (or principal) nucleus, which dves 



Edinger-Westphal nucleus. 

 Principal nucleus. 

 Median nucleus. 



Nucleus of 4th nervt. 



Fig. 107.— Nuclei of origin of the third and fourth nerve.s. — (From Poirier and Charpy.) 



origin chiefly to the fibers innervating the extrinisc muscles; the 

 median nucleus; and the nucleus of Edinger-Westphal. According 

 to Bernheimer* the large median nucleus gives rise to the fibers 

 that innervate the ciliary muscles, while the Edinger-Westphal 

 nuclei (accessory nuclei) control the movements of the sphincter 

 muscle of the iris. Some of the fibers, particularly those from 

 the lateral nucleus to the inferior rectus, the internal rectus, and 

 the inferior oblique, cross the midline and emerge in the nerve 

 of the opposite side. 



The Fourth Cranial Nerve ( A^. Trochlearis) . — This nerve emerges 



* Bernheimer, in "Graefe-Saemisch's Handbuch der ges. Augenheillvunde," 

 2d ed., I, 41. 



