784 



DISSECTION OF THE CEEEBELLDM. 



floor of 

 fourth 

 ventricle. 



In lower 



half of floor. 



four nuclei, 



viz., 



of twelfth, 



tenth, 

 ninth, 



and eighth 

 nerves. 



Beneath 

 upper half 

 are five 

 nuclei, viz., 



of sixth, 



seventh, 

 two of fifth, 



In midbrain 

 three nuclei 

 viz., of 

 third, 

 fourth, and 

 fifth nerves. 



Some of these appear in the floor of the fourth ventricle, while 

 others are placed a little below the surface. 



Beneath the calamus scriptorius, and in the portion of the floor 

 of the cavity formed by the medulla oblongata, are the nuclei of the 

 twelfth, tenth, ninth and eighth nerves. The hypoglossal nucleus 

 (XII) extends through nearly the whole length of the medulla 

 oblongata ; its lower part is hidden, lying in front of the central 

 canal below the level of the fourth ventricle, but its upper half 

 approaches the surface in the lower portion of the fasciculus teres. 

 The main nucleus of the vagus (X) corresponds to the ala cinerea, 

 and is continued above into the chief glosso-pharyngeal nucleus (IX) 

 which lies beneath the inferior fovea. The position of the inner or 

 principal nucleus of the vestibular division of the auditory nerve 

 (VIIK) is indicated by the area acustica ; and the dorsal nucleus of 

 the cochlear division (VIIIo) by an enlargement just above the 

 acoustic striae. 



In the hinder part of the pons, and beneath the upper half of the 

 floor of the ventricle, are nuclei of the fifth (two), sixth, seventh 

 and eighth nerves, but only that of the sixth is indicated by a 

 superficial prominence. The nucleus of the sixth (VI) lies beneath 

 the elevation of the fasciculus teres above the auditory striae. The 

 facial nucleus (VII) is placed external to and deeper than the last ; 

 and the motor nucleus of the fifth nerve (Vra) is above that of the 

 facial. The sensory nucleus of the fifth (Vs) lies external to the 

 foregoing, and just beyond the lateral margin of the ventricle. 



Above the fourth ventricle, in the grey matter surrounding the 

 aqueduct of Sylvius, are the nuclei of the fourth (IV) and third (III) 

 nerves in the floor, and the nucleus of the descending root of the fifth 

 (Vd) on the side of the canal. 



Further details of these nuclei are given on pages 728 to 730. 



