THE CRANIAL NERVES AND THEIR NUCLEI 



173 



V 



The nucleus of the trochlear nerve has already been located in the central 

 gray matter ventral to the cerebral aqueduct at the level of the inferior collic- 

 ulus, close to the caudal extremity of the oculomotor nucleus (Figs. 114, 121, 

 122). The fibers of the trochlear nerve emerge from the dorsal and lateral 

 aspects of this nucleus, and, encircling the central gray matter along an angular 

 course which carries them also caudally, enter the anterior medullary velum, 

 decussate within it, and make their exit from its dorsal surface (Fig. 112). They 

 supply the superior oblique muscle. 



The nucleus of the abducens nerve was encountered in the dorsal portion 

 of the pons as a spheric gray mass, which with the genu of the facial nerve forms 

 the facial colliculus of the rhomboid fossa (Figs. 

 108, 121, 122). The fibers of the abducens nerve 

 leave the nucleus chiefly on its dorsal and medial 

 surfaces and become assembled into several root 

 bundles, which are directed ventrally toward their 

 exit from the lower border of the pons near the 

 pyramid of the medulla oblongata. It supplies 

 the lateral rectus muscle. 



The axons, which ramify within the three nuclei 

 for the motor nerves of the eye, are derived from 

 many sources. The most important of these 

 sources are the corticobulbar tract, the medial lon- 

 gitudinal bundle, and the tectospinal tract. The 

 nucleus of the abducens receives fibers also from 

 the central auditory apparatus through the pe- 

 duncle of the superior olive. These various fibers 

 provide for voluntary movements of the eyes, and 



for reflex ocular movements in response to vestibular, visual, and auditory 

 impulses. The nuclei probably also receive branches from the central sensory 

 path of the fifth nerve. 



The nucleus of the hypoglossal nerve is a slender cylindric mass of gray 

 matter nearly 2 cm. in length, extending from the level of the fovea inferior to 

 that of the decussation of the pyramids. We have already identified it in both 

 the open and the closed portions of the medulla oblongata (Figs. 99, 103). In the 

 floor of the fourth ventricle it lies beneath the trigonum hypoglossi, while more 

 caudally it lies ventral to the central canal (Figs. 121, 122). The root fibers 



Fig. 123. Diagram of the 

 nuclei of the oculomotor nerve: 

 M, Median nucleus; E.W., nu- 

 cleus of Edinger-Westphal; V.L., 

 D.L., ventral and dorsal portions 

 of the lateral nucleus. (Ober- 

 steiner.) 



