THE NERVOUS SYSTEM 1367 



fifth nerve, and the fibres of that root terminate at intervals in 

 arborizations around its cells. 



The posterior longitudinal bundle has the same position in the 

 upper region as it has in the lower region. It lies close to the 

 median raphe and immediately below the grey matter of the corre- 

 sponding part of the floor of the fourth ventricle. 



The main or mesial fillet, like the main fillet in the lower region, 

 lies in that portion of the upper region of the dorsal part of the pons 

 which is contiguous to the ventral part, and it forms a layer of 

 some breadth, extending outwards from the median raphe. 



The lateral fillet represents a strand of fibres which lies on the 

 lateral or outer side of the mesial fillet. Associated with the lateral 

 fillet, and lying between it and the mesial fillet, there is a collection 

 of nerve-ceils, called the nucleus of the lateral fillet. 



Development of the Pons Varolii. — The pons is developed from the ventral 

 and lateral walls of the metencephalon, which is one of the divisions of the 

 rhombencephalon. 



3. The Cerebellum. 



The cerebellimi, or small brain, occupies the inferior occipital 

 or cerebellar fossae of the occipital bone. It lies beneath the 

 posterior parts of the hemispheres of the cerebrum, from which it 

 is separated by a septum of the dura mater, called the tentorium 

 cerebelli, and it is posterior to the medulla oblongata and pons 

 Varolii. It is composed of white and grey matter, the white 

 matter being situated in the interior, where it constitutes the 

 medullary substance, and the grey matter being spread over the 

 surface as the cortex. In appearance it is laminated or foliated, 

 the laminae being separated from each other by parallel, slightly 

 curved sulci. It is composed of t^vo large lateral portions, called 

 hemispheres, and a connecting median portion, termed the vermis 

 (■ worm ') or vermiform process, these parts being much more 

 distinctly demarcated below than above. It presents, in the 

 median line, two notches, anterior and posterior. The anterior 

 notch, which is wide, is known as the incisura semilunaris, and it 

 contains the inferior pair of quadrigeminal bodies and the superior 

 cerebellar peduncles. The posterior notch is narrow, and is occupied 

 by the falx cerebelli. The most conspicuous sulcus of the cere- 

 bellum is the great horizontal fissure, which extends round the 

 circumference, and passes for some distance into the interior. By 

 means of this fissure the cerebellum is divided into two parts, upper 

 and lo\\'er. 



Upper Surface of the Cerebellum. — ^This surface presents in the 

 median line the upper part of the vermis, known as the superior 

 vermis. It extends from the incisura semilunaris to the posterior 

 notch, and it forms a laminated elevation, which is higher in front 

 than behind, the most prominent part being known as the monticulus 

 cerebelli. On either side of the superior vermis the upper surface 

 of each hemisphere inclines downwards to the circumference, and 

 there is no distinct demarcation between it and the superior vermis. 



