230 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



Fig. 100, namely, the lateral (or principal) nucleus, which gives 

 origin chiefly to the fibers innervating the extrinsic 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 mid-line and emerge in the nerve 

 of the opposite side. 



Edinger-Westphal nucleus. 

 Principal nucleus 

 Median nucleus. 



Nucleus of 4th nerve. 



Fig. 100. Nuclei of origin of the third and fourth nerves. (From Poirier and Charpy.) 



The Fourth Cranial Nerve (N. Trochlearis) . This nerve emerges 

 from the brain in the anterior medullary velum (valve of Vieussens) 

 just posterior to the inferior colliculus. It curves around the crus 

 cerebri to reach the base of the brain. It is a motor nerve, and 

 supplies fibers to the superior oblique muscle of the eyeball. In 

 the interior of the brain the fibers arise from a nucleus in the central 

 gray matter just posterior to that of the third nerve (Fig. 100). 

 The fibers pass dorsalward toward the velum and make a com- 

 plete decussation before emerging. 



The Fifth Cranial Nerve (N. Trigeminus). This nerve arises 

 from the side of the pons by two roots, a small motor root, portio 



* Bernheimer, in " Graefe-Saemisch's Handbuch der ees. Augenheilkunde." 

 2ded., I., 41. 



