THE PONS VAROLII. 



1079 



Portion of cross-section of pons, showing cells of pontine 



nucleus. X 300. 



(nucleus pontis) is distributed within the interstices between the bundles of nerve- 

 fibres. The cells of this nucleus, small in size and stellate in form, are closely 

 related to the ponto-cerebellar fibres of the same and of the opposite side, many 

 constituting stations of interruption in the cortico-cerebellar paths. 



The dorsal or tegmental part of the pons (pars dorsalis pontis) resembles 

 to a considerable extent in its general structure the formatio reticularis grisea of 

 the medulla, consisting for the most part of a reticulum of transverse and longitudinal 

 fibres, interspersed with nerve-cells, on each side of the median raphe. The appear- 

 ance of certain new masses 



of gray matter and of nerve- -^^^^ 934- 



fibres, together with changes 

 in the position of the fillet, 

 produce details that vary 

 with the level of the section. 

 When this passes above the 

 lower margin of the pons 

 (Fig. 933). two diverging 

 and obliquely cut strands of 

 fibres, coursing from the 

 ventricular floor towards the 

 ventral aspect, mark the root- 

 fibres of the sixth and seventh 

 cranial nerves and divide the 

 dorsal region, on each side, 

 into three areas. The middle 

 area, between the abducent 

 fibres mesially and the facial 

 fibres laterally, contains three 

 important collections of nerve- 

 cells. One of these, the nu- 

 cleus of the sixth nerve, lies close to the floor of the ventricle and beneath the 

 rounded prominence of the eminentia teres, which it helps to produce, and gives 

 origin to the root-fibres of the abducent ner\-e. These fibres take an obliquely 

 ventral path, slightly bowed towards the raphe, and cut through not only the dorsal 

 but also the ventral part of the pons to gain its lower border, along which they 

 emerge a few millimeters from the mid-line. In favorable sections the nucleus of the 

 sixth is seen separated from the floor of the fourth ventricle by the arching fibres of 

 the facial nerve. 



Another conspicuous nucleus of the middle area, the superior olive (nucleus 

 olivaris superior), lies near the ventral limit of the tegmental area, partly lodged within 

 an indentation on the dorsal surface of the conspicuous tract of transverse fibres, 

 known as the corpus trapezoideum, that extends from the ventral cochlear 

 nucleus medially and materially aids in defining the ventral boundary of the dorsal 

 area. The superior olive (Fig. 933) is an irregularly spherical collection of nerve-cells, 

 interposed in the path connecting the auditory nuclei with the cerebral cortex, and 

 closely related with the tract of the lateral fillet (page 1082). In addition to contrib- 

 uting numerous fibres to the latter, the superior olive sends others to the abducent 

 nucleus which are seen as delicate strands, the peduncle of the superior olive, that 

 pass towards the nucleus of the sixth nerve and bring this centre into relation with 

 auditory impulses. A small collection of nerve-cells between the fibres of the trape- 

 zoidal tract, ventro-medial to the superior olive, constitutes the 7iucleus trapezoideum. 

 Close to the medial border of the superior olive a small oval bundle of longitudinal 

 fibres, the central tegmental fascicubis, is sometimes seen. These fibres are probably 

 derived from the olivary nucleus (Obersteiner). 



The facial nucleus, a conspicuous but broken oval mass of gray matter 

 (Fig. 933), includes several groups of large stellate cells that lie dorso-lateral to 

 the superior olive and to the inner side of the emerging facial fibres. From the 

 cells of this nucleus the loosely collected root-fibres of the facial nerve pass back- 

 ward and inward to reach the floor of the fourth ventricle. Here they converge into 



