CEREBELLUM, PONS, AND MEDULLA. 



247 



the inferior oblique, cross the mid-line and emerge in the nerve 

 of the opposite side. 



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 

 pedunculus 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. 107). The fibers pass dorsal ward toward the 

 velum and make a complete decussation before emerging. 



Edinger-Westphal nucleus. 

 Principal nucleus 

 Median nucleus. 



Nucleus of 4th nerve. 



Fig. 107. Nuclei of origin of the third and fourth nerves. (From Poirier and Charpy.) 



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, 

 portions of the ear, mouth, and tongue, and to the dura mater 

 (Fig. 108). In the interior of the brain the motor portion, portio 

 minor, arises partly from a small nucleus in the pons and partly 

 from a long column of cells extending along the lower margin of the 

 central gray matter throughout the midbrain. This column and 

 the fibers arising from it constitute the descending motor root of the 



