1362 A MANUAL OF ANATOMY 



2, The Pons Varolii. 



The pons Varolii is situated above the medulla oblongata, anc 

 between the hemispheres of the cerebellum. With the exceptior 

 of the restiform bodies, all the other parts of the medulla oblongata 

 are prolonged into it. The pons presents two surfaces, ventral anc 

 dorsal, and two borders, upper and lower. The ventral surfacf 

 rests upon the upper part of the basilar groove of the occipital bon( 

 and the dorsum sellse of the sphenoid. It is convex from side tc 

 side, and from above downwards, and has a transversely-striatec 

 appearance, due to the disposition of its superficial fibres. Alon^ 

 the median line it presents the basilar groove, which extends fron 

 the lower to the upper border, and lodges the basilar artery. Or 

 either side of this groove the ventral surface is rendered prominen- 

 by the prolongation upwards of the pyramids of the medulli 

 oblongata, and the basilar groove is chiefly due to this circumstance 

 1 he sensory and motor roots of the fifth nerve, lying close together 

 appear on the lateral aspect of the ventral surface, the small moto: 

 root being the upper of the two. The portion external to these 

 two nerve-roots constitutes the middle peduncle of the cerebellum 

 It is composed of the transverse fibres of the pons, these on eithei 

 side being collected into a large bundle, which passes backward: 

 and outwards into the corresponding cerebellar hemisphere. 



The dorsal surface is directed towards the cerebellum. II 

 presents a triangular area which is covered with grey matter. This 

 area is continuous with the dorsal surface of the upper or open pari 

 of the medulla oblongata, and it forms the upper or pontine pari 

 of the floor of the fourth ventricle. On either side it is boundec 

 by the superior peduncle of the cerebellimi as it passes upwards anc 

 inwards. 



The upper border is slightly depressed at the centre, and on eithei 

 side of the median depression it slopes outwards and downwards 

 towards the middle peduncle of the cerebellum. The crura cerebri 

 right and left, sink into the pons at the upper border. 



Internal Structure of the Pons Varolii. — The pons is composed 01 

 a large ventral and a small dorsal part. 



Ventral Part. — This portion consists of: (i) bundles of transversa 

 fibres, (2) bundles of longitudinal fibres, and (3) a large amount ol 

 grey matter. 



The bundles of transverse fibres intersect the bundles of longi- 

 tudinal fibres, and on either side they are collected into the middle 

 peduncle of the cerebellum, which enters the corresponding cere- 

 bellar hemisphere. Some of the transverse fibres arise in the cortex 

 of the cerebellum as the axons of the cells of Purkinje, and these 

 terminate in the pons in arborizations around the cells of the 

 nucleus pontis, mostly on the opposite side to that on which thej 

 arise. Other transverse fibres arise in the pons as the axons of the 

 cells of the nucleus pontis on one side. They then cross to the other 

 side, and enter the cerebellar hemisphere of that side, where thej 



