THE PONS VAROLII. 



1077 



tract of gray matter that collectively constitutes the reception-nucleus of the cochlear division of 

 the auditory nerve. This ganglion is subdivided into a superior and an inferior portion, these 

 being the dorsal cochlear tiucleus and the ventral cochlear nucleus respectively. They both 

 receive the fibres of the cochlear or lateral division of the auditory nerve. The ventral cochlear 

 nucleus is the starting point of a tract of transverse fibres, that pass horizontally inward, many 

 traversing the fillet and crossing the raphe, and intermingle with those from the opposite 

 side. They thus form a broad strand, the corpus trapezoides, that within the pons occupies 

 the lower limit of the tegmental region, which it separates from the ventral. In Fig. 932 



Fig. 932. 



Descend- Gray substance 

 Substantia ing root of floor 



gelatinosa ofVIII ofventricle 



Mesial vesliliular 

 nucleus 



Deiters' nucleus 



Post. long, 

 fasciculus 



Vestibilar ner\e 



Restiforni 

 body 



Cochlear nerve 



and ventral cochlear nucleus / 

 Spinal root of V 

 Trapezoidal fibres' 

 Inferior olivarj- nucleus 



Median fillet 







Restiform body 



Cochlear nerve 



Dorso-lateral 

 ^^7^ cochlear 

 ,i nucleus 



i 

 I 



Ventral cochlear nucleus 



Pyramidal tract ' 



Transverse section of medulla at level H, Fig. 919; pyramids are small and inferior olivary nuclei are disappearing; 

 roots of auditory nerve are entering in relation to restiform bodies. X 4. Preparation by Professor Spiller. 



only the beginning of this tract is visible, but slightly higher, in the pons (Fig. 933), the 

 trapezoidal fibres are shown in force. Strands of fibres from the cochlear nuclei arch over the 

 restiform body and proceed beneath the ventricular floor to the mid-groove ; these mark the 

 course of the slrit^ acusticcs seen crossing the ventricle. Ventro-mesial to the spinal root 

 of the trigeminus and the associated Rolandic substance the nucleus 0/ the facial nerve appears 

 as an irregularly oval and somewhat broken group of large stellate cells, from which the 

 strands of root-fibres pass dorso-medially. 



THE PONS VAROLII. 



Viewed from in front, the pons appears as a quadrilateral prominence on the 

 ventral aspect of the brain, interposed between the medulla oblongata below, the 

 cerebral peduncles above, and the cerebellar hemispheres at the sides. Its lower 

 and upper limits are well defined by grooves that separate the corresponding borders 

 from the adjacent divisions of the brain -stem, and between these boundaries the pons 

 measures from 25-28 mm. in the mid-line. Laterally, however, its limits are 

 unmarked, as here the mass of the pons narrows and is directly continued on each 

 side as a robust arm which sweeps downward and backward into the cerebellum as 

 the middle cerebellar peduncle. The fibres of the trigeminal nerves, which are 

 attached near its upper and lateral margins, are taken as the conventional lateral 

 limits of the pons, the transverse diameter measured between these points being 

 about 30 mm. 



The ventral surface of the pons, strongly convex transversely and less so in 

 the opposite direction, lies behind the basilar process of the occipital bone and the 

 dorsum sellse. It is marked by a shallow median groove (sulcus basilaris), which 

 broadens as it ascends and lodges the basilar artery and is bounded on each side by a 

 slight longitudinal elevation. Where the latter meets the medulla, the pyramid is seen 

 to plunge into the pons beneath its transversely striated surface. The longitudinal 



