THE FOUKTH VENTBICLE. 549 



>posite the lateral border of the pyramid ; the facial and acoustic are also attached 

 i the inferior edge of the pons, but far away from the median plane. The acoustic 

 ?rve is in contact with the cerebellum and the facial is on its medial side, with its 

 nsory root (the nervus interrnedius) between them (Figs. 478, 481). A large bundle 

 ' fibres upon the front of the pons departs from the transverse course pursued by 

 .ost of the pontine fibres, and starting at the medial side of the trigeminal nerve, 

 isses almost vertically downwards between the facial and acoustic nerves (Fig. 

 26, p. 593) and reaches the lateral aspect of the medulla oblongata, where it passes 

 Lto the corpus ponto-bulbare (Fig. 481). This bundle is known as the fasciculus 

 )liquus [pontis]. It is interesting to observe that while the facial nerve lies upon 

 le medial side of this oblique bundle, its sensory root (the nervus interrnedius) is 

 aced on its lateral aspect, alongside the acoustic nerve (Fig. 527, p. 594). 



Immediately below the insertion of the acoustic nerve at the interior margin of 

 le pons a little calyx-like appendage of the epithelial roof of the fourth ventricle 

 ecessus lateralis) projects laterally, partly behind the glossopharyngeal nerve, 

 hrough an elliptical aperture in this epithelial process (apertura lateralis 

 jntriculi quarti) a little cauliflower-like mass of chorioid plexus becomes extruded 

 itween the acoustic and the glossopharyngeal nerves (Fig. 527, p. 594). 



The posterior surface of the pons looks backwards towards the cerebellum, and 

 :esents a triangular area covered with gray matter, which forms the superior part 

 ' the anterior wall or floor of the fourth ventricle. This area is directly continuous 

 tferiorly with the medullary part of the floor of the fourth ventricle, and is bounded 

 i each side by a band of white matter termed the brachium conjunct! vum (Fig. 482). 



Brachia Conjunctiva (O.T. Superior Peduncles of the Cerebellum). The 

 rachia conjunctiva are hidden from view by the superior part of the cerebellum, 

 nder cover of which they lie. They emerge from the hemispheres of the cerebellum, 

 id, as they proceed upwards on the dorsal aspect of the pons, they converge towards 

 ich other until, at the inferior level of the corpora quadrigemina, the medial 

 targins of the two brachia become almost contiguous (Fig. 482, p. 550). At first 

 ley form the lateral boundaries of the superior part of the fourth ventricle ; but, 

 ,i they ascend and approach closer to each other, they gradually come to overhang 

 mt cavity, and thus enter into the formation of its roof. They disappear from 

 le surface of the brain by dipping under cover of the quadrigeminal bodies and 

 itering the substance of the mesencephalon. 



Velum Medullare Anterius. Filling up the triangular interval between the two 



rachia conjunctiva, and stretching across from the medial and free margin of the 



le to the corresponding margin of the other, is a thin layer of white matter which 



)mpletes the roof or dorsal wall of the upper part of the fourth ventricle, and 



iceives the name of the anterior medullary velum. When traced downwards, the 



^lum is seen to be carried, with the brachia conjunctiva, into the white matter of 



.le cerebellum. Spread out on its posterior or superior surface is a small, thin, 



>ngue- shaped prolongation of gray matter from the cortex of the cerebellum, 



hich is termed the lingula, whilst issuing from its substance close to the inferior 



uadrigeminal bodies are the two trochlear nerves. 



VENTRICULUS QUARTUS. 



Fourth Ventricle. The fourth ventricle of the brain is somewhat rhomboidal in 

 >rm. Below, it tapers to a point and becomes continuous with the central canal 



the lower half of the medulla oblongata ; above, it narrows in a similar manner 

 nd is continued into the aquseductus cerebri, which tunnels the mesencephalon. 

 he posterior wall is termed the tegmen or roof and is concealed by the cerebellum, 

 'he anterior wall is called the floor and is formed by the dorsal surface of the 

 ons and the corresponding surface of the medulla oblongata. On each side a long, 

 ,urved and narrow prolongation of the ventricular cavity is carried laterally from 



i widest part and curves round the upper part of the corresponding restiform 

 ,ody. This is termed the recessus lateralis. The roof of the cavity is very thin 

 nd is intimately connected with the cerebellum. It is better, therefore, to defer 

 ;s description until that part of the brain has been studied. 



365 



