THE CRANIAL NERVES AND THEIR NUCLEI 



185 



to the muscles of mastication. If this interpretation is correct we are pre- 

 sented with an exception to the rule that the afferent fibers of the cerebrospinal 

 nerves take origin from cells located outside the cerebrospinal axis. This nucleus 

 lies in the lateral wall of the rostral portion of the fourth ventricle and in the 

 lateral part of the gray matter surrounding the cerebral aqueduct (Figs. 114, 

 121, 130). The origin and termination of the afferent fibers for the extrinsic 



Fig. 132. Diagram to show the location of the secondary sensory tracts of the trigeminal 

 nerve (solid black) in the tegmental portion of the rostral part of the pons: B.C., Brachium con- 

 junctivum; D.T.T.N., dorsal secondary sensory tract of the trigeminal nerve; L.L., lateral lemnis- 

 cus; M. L., medial lemniscus; M.L.F., medial longitudinal fasciculus; V.T.T.N., ventral secondary 

 sensory tract of trigeminal nerve. 



muscles of the eye are unknown, although we know that such afferent fibers 

 are present in the oculomotor, trochlear, and abducens nerves. 



SPECIAL SOMATIC AFFERENT NUCLEI 



The special somatic afferent nuclei are associated with the acoustic nerve, 

 which is composed of two divisions. One part, the cochlear neroe, conveys im- 

 pulses aroused by sound waves reaching the cochlea through the outer ear 

 and tympanic cavity. Since it responds to stimuli from without, the cochlear 

 apparatus subserves exteroceptive functions. The vestibular neroe, on the other 

 hand, conveys impulses from the semicircular canals of the ear. These are im- 

 portant proprioceptive sense organs and give information concerning the move- 

 ments and posture of the head. 



The cochlear nuclei are the terminal nuclei of the cochlear nerve, the fibers 

 of which take origin in the spiral ganglion of the cochlea. This is composed of 

 bipolar cells, each having a short peripheral and a longer central process (Fig. 

 133). The peripheral process terminates in the spiral organ of Corti. The 

 central process is directed toward the brain in the cochlear nerve. These central 

 fibers terminate in two masses of gray matter, located on the restiform body near 

 the point where the latter turns dorsally into the cerebellum (Figs. 107, 121, 



