CEREBELLUM, PONS, AND MEDULLA. 



227 



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. Some of 

 the fibers, particularly those from the lateral nucleus to the infe- 

 rior rectus, the internal rectus, and the inferior oblique, cross 

 the mid-line and emerge in the nerve of the opposite side. 



The Fourth Qranial 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 cms 

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



Nucleus of com. 



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.) 



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 

 minor, and a large sensory root, portio major. It is, therefore, a 

 mixed motor and sensory nerve, supplying motor fibers to the 

 muscles of mastication and sensory fibers of pressure, pain, and 

 temperature to the face, the forepart of the scalp, the eye, nose, 



