THE PONS VAROLII. 451 
form body, whilst the auditory nerve reaches the brain close to the facial nerve, on 
the ventral aspect of the restiform body. 
The whole of the medulla enters the lower aspect of the pons, and, with the 
exception of the restiform bodies, its constituent parts are, to a large extent, carried 
up within it. The crura cerebri emerge from its upper aspect. 
The dorsal surface of the pons is turned backwards towards the cerebellum, and 
presents a triangular area covered with gray matter, which forms the upper part of 
the anterior wall or floor of the fourth ventricle. This area is directly continuous 
below with the medullary part of the floor of the fourth ventricle, and is bounded 
on either side hy a band of white matter termed the superior peduncle of the 
cerebellum. 
The superior cerebellar peduncles (brachia conjunctiva) are hidden from 
view by the upper part of the cerebellum, under cover of which they lie. They 
emerge from the lateral hemispheres of the cerebellum, and, as they proceed 
upwards on the dorsal aspect of the pons, they converge towards each other until, 
at the level of the inferior corpora quadrigemina, the inner margins of the two 
peduncles almost become contiguous (Fig. 335, p. 447). At first they form the 
lateral boundaries of the upper part of the fourth ventricle; but, as they ascend 
and approach closer to each other, they gradually come to overhang that cavity 
and thus enter into the formation of its roof. They disappear from the surface 
by dipping under cover of the quadrigeminal bodies and entering the substance of 
the mesencephalon. 
Valve of Vieussens or the Superior Medullary Velum (velum medullare 
anterius).—Filling up the triangular interval between the two superior cere- 
bellar peduncles, and stretching across from the inner and free margin of the one 
to the corresponding margin of the other, is a thin layer of white matter which 
completes the roof or dorsal wall of the upper part of the fourth ventricle, and 
receives the name of the superior medullary velum. When traced downwards, it 
is seen to be carried with the superior peduncles into the white matter of the cere- 
bellum. Spread out on its dorsal surface is a small, thin, tongue-shaped prolongation 
of gray matter from the cortex of the cerebellum, atin is termed the lingula, 
whilst issuing from its substance close to the inferior quadrigeminal bodies are the 
two fourth or trochlear cranial nerves. 
Fourth Ventricle of the Brain (ventriculus quartus)—The fourth ventricle is 
somewhat rhomboidal in form. Below, it tapers to a point and becomes con- 
tinuous with the central canal of the cord; above, it narrows in a similar manner 
and is continued into the aqueduct of Sylvius, which tunnels the mesencephalon. 
The posterior wall is termed the roof and is concealed by the cerebellum. The 
anterior wall is called the floor and is formed by the dorsal surface of the medulla 
- and pons. On either side a narrow-pointed prolongation of the ventricular cavity 
is carried outwards from its widest part and curves round the upper part of the 
corresponding restiform body. This is termed the lateral recess. The roof of 
the cavity is very thin and intimately connected with the cerebellum. It is better, 
therefore, to defer its description until that part of the brain has been studied. 
é Floor of the Fourth Ventricle (fossa rhomboidea).—In its lower part the 
floor of the fourth ventricle is formed by the dorsal surface of the open part of the 
medulla, whilst in its upper part it is formed by the dorsal surface of the pons 
Varoli (Fig. : 333, p. 447). The area thus constituted is lozenge-shaped, its widest 
part beimg opposite the middle peduncles of the cerebellum. A thick layer of 
| gray matter, continuous with that which surrounds the central canal of the cord, 
1s spread out like a carpet over the ventricular floor, and covering this is the usual 
ependymal layer, which lines all the ventricles of the brain. The area is circum- 
scribed by definite lateral boundaries. Thus, below it is bounded on either side by 
the clava, the cuneate tubercle, and the restiform body; whilst above the lateral 
limits are formed by the superior cerebellar peduncles. 
The floor of the fourth ventricle is divided into two lateral and symmetrical 
portions by a median groove. At the lower narrow end between the two clave it 
receives the name of the calamus scriptorius, from its fancied resemblance to the 
point of a pen. Crossing each half of the floor, at its widest part, are several trans- 
ade 
