476 NEUROLOGY. 



This valve covers the aqueduct of Sylvius, and, with the vermi- 

 form process, forms the roof of the fourth ventricle. The- 

 posterior vermiform process is not so well marked. The smaller 

 tissures, or sulci, divide the cerebellum into numerous lobules, 

 which, when separated, are found to be indented with numerous 

 secondary fissures. Each lobule consists of a periphery of grey 

 matter, and a ramification of white fibres from the centre, so 

 that the whole organ presents an arborescent appearance when 

 cut across, an appearance which has gained the name of the arbor 

 vitce. The centre of the cerebellum contains a mass of white 

 matter, within which is a denticulated capsule of grey matter 

 (corpus dentatum), similar to that of the olivary bodies, from 

 which white fibres proceed to the peduncles and to the valve of 

 Vieussens, the centre white mass consisting of three bundles of 

 fibres, each of which proceeds to a separate peduncle. 



FOURTH VENTRICLE. 



The fourth ventricle is the lozenge-shaped space between the 

 cerebellum and the medulla, its roof being formed by the vermi- 

 form process and valve of Vieussens, its lateral walls by the 

 superior pyramids, restiform bodies, and anterior peduncles of 

 the cerebellum, and its floor by the medulla and pons Varolii ; 

 the testes being its boundary anteriorly, and the restiform bodies 

 posteriorly. A fissure, continuous with the superior median 

 fissure of the cord, passes along the floor of the ventricle into the 

 aqueduct of Sylvius, which is a canal leading into the third ven- 

 tricle ; and on either side of this fissure a few fibres, which join 

 the auditory nerve, unite at an angle somewhat resembling the 

 barb of a pen, and constitute what is known as the calamus 

 scriptorius. The lining membrane of this ventricle is continuous 

 with that of the others, through the aqueduct of Sylvius, and 

 v/ith the subarachnoidean space ; it forms a small fringe known 

 as the choroid plexus of the fourth ventricle. 



CEREBRUM. 



The cerebrum, or great brain, occupies the anterior portion of 

 the cranial cavity. It is ovoid in shape, with an irregularly 

 flattened base, and consists of lateral halves or hemispheres, 

 separated by the longitudinal fissure, in which the falx cerebri is. 



