THE NERVOUS SYSTEM 1365 



are so complicated that it is convenient to divide the dorsal part of 

 the pons into two regions — ^lower and upper. 



Lower Region. — ^This region corresponds to the level of the trape- 

 zium in the ventral part of the pons, and it succeeds to the upper 

 end of the bulb. The restiform body of the bulb lies for a short 

 distance on the outer aspect of this region, but that body soon 

 passes backwards and sinks into the hemisphere of the cerebellum. 



The tracts and nuclei of the formatio reticularis of the lower 

 region, which will be described in this place, are as follows: 



1 . Spinal sensory root of the fifth cranial nerve. 



2 . Motor nucleus of the facial nerve. 



3. Superior olivary nucleus. 



4. Nucleus of the sixth cranial nerve. 



5. Dorsal longitudinal bundle. 



6. FiUet. 



The funiculi of the spinal or descending sensory root of the fifth 

 cranial nerve appear on the inner side of the restiform body. Close 

 to the inner side of this root, and accompanying it in its downward 

 course, there is the inferior sensory nucleus of the fifth nerve, 

 around the cells of which the fibres of the spinal or descending root 

 of that nerve terminate at intervals. The inferior sensory nucleus 

 is an upward prolongation of the substantia gelatinosa of Rolando, 

 and inferiorly it extends to about the level of the second cervical 

 spinal nerve. 



The motor nucleus of the facial nerve is internal to the funiculi 

 of the spinal root of the fifth nerve. It lies deeply in the lower 

 region of the dorsal part of the pons, on the dorsal aspect of the 

 superior olivary nucleus. The motor fibres of the facial nerve 

 arise as the axons of the cells of this nucleus, and take a course 

 which will be afterwards described. 



The superior olivary nucleus is situated on the ventral aspect of 

 the facial nucleus, and is close to the lateral part of the trapezium, 

 the fibres of which arch round its ventro-lateral aspect. Some of 

 these fibres terminate in the superior olivary nucleus, whilst others 

 arise from its cells. 



In connection with the superior olivaxy nucleus three other nuclei are 

 described — namely, (i) an accessory superior olivary nucleus on the mesial 

 side of the principal nucleus; (2) a lateral pre-olivary nucleus on the ventral 

 aspect of the principal nucleus; and (3) a mesial pre-olivary nucleus on the 

 ventral aspect of the nucleus of the trapezium. 



The nucleus of the sixth cranial nerve lies immediately beneath 

 the grey matter of the pontine part of the floor of the fourth ven- 

 tricle, and on the outer side of the dorsal longitudinal bundle which 

 separates the nucleus from the median raphe. It corresponds to 

 that portion of the eminentia teres which lies on the pontine part of 

 the floor of the fourth ventricle directly above the striae acusticge. 



The posterior longitudinal bundle, or median longitudinal fasciculus 



