FOURTH VEXTRICLE 717 



ivstiform body to the superior surface. The restiform body T inferior peduncle) is 

 joined on entering the cerebehuni by the inferior fibres of the jions; the two together 

 unite into a distinct round bundle, the inner part of which is formed by the resti- 

 form 1)ody ; the outer by the pons-tibres. This bundle passes through a distinct gap 

 or interval bounded by the sui)erior peduncle internally, and by the greater part of 

 the middle j)eduncle externally. Its fibres then spread out into a layer, the fibrte 

 semicirculares of Stilling which are disposed in gentle curves convex backwards 

 and outwards; these fibres cover the capsule of the corpus dentatum, and finally 

 sweep abruptly upwards into the folia of the superior vermis. Some fibres enter 

 the nucleus fastigii. The fibres of the inferior peduncle are connected with the 

 opposite olivary body, Avith the fillet and posterior longitudinal bundle of the 

 same side, and with the antero-lateral column of the cord. 



The corpus dentatum is covered by a distinct capsule of fibres (the ' fleece ' of 

 Stilling) derived from the olivary nucleus of the opposite side. The fibres are 

 prol)ably for the mr)st part interrupted in the nerve-cells of the corpus dentatum, 

 and are then (indirectly) continued into the white matter Avhich occupies the 

 interior of that plicated bag of grey matter. From the hilum of the corpus 

 dentatum a bundle of fibres passes out, which is joined by fibres from the fleece, 

 and thus the superior cerebellar peduncle is fomied. The superior cerebellar 

 peduncle passes upwards and inwards, and receives a covering on its outer side 

 from the inferior fillet. It disappears from the surface by passing under cover of 

 the testis of its own side, and assumes a deeper position in the mesencephalon, 

 jjassing to the ventral side of the aqueduct of Sylvius. The majority of the fibres 

 decussate with their fellows of the opposite side, the decussation being most pro- 

 nounced at the level of the centre of the nates. The decussation l^eing com- 

 pleted, the decussated fibres and those which remain on the same side proceed 

 brainwards as a distinct column. This column of fibres soon encounters the red 

 nucleus, and the fibres pass through the nucleus in small bundles, the cross-section 

 of which gives the punctated appearance to the red nucleus w^hich is so charac- 

 teristic of that body when seen in coronal section. Beyond the red nucleus, fibres 

 <»f the superior peduncles certainly enter the optic thalamus, and proljably the 

 globus pallidus and the cortex cerebri. 



(6) Sagittally-directed fibres are placed within the superior vermis. When 

 traced forwards these fibres are found to decussate with their fellows of the opposite 

 side in front and above the nucleus fastigii; here they are joined by true commis- 

 sural fibres from its hemispheres, the whole forming the great anterior cerebellar 

 commissure of Stilling. 



Filires unite the different folia to one another, constituting a system of fibres 

 which form the most peripheral ]k\v{ <>f the arbor vitte. These fibres, from their 

 general arrangement, are called garland-like fasciculi. 



The FOURTH VENTRICLE is a cavity which is Avidest and most lofty about its 

 centre, and narrowed at its anterior and posterior extremities. It communicates 

 with the third ventricle al)ove through the aqueduct of Sylvius, and passes 

 below into the central canal of the spinal cord. Its long axis is directed upwards 

 and forwards. The structures Avhich bound it behind are said to constitute its 

 roof, while the parts bounding it in front are described as its floor. The roof is 

 formed from above doAvnwards Ijv the folloAving structures: the superior peduncles 

 of the cerebellum with the valve of Vieussens between them, the Avhite matter of 

 the vermiform process of the cerebellum, the inferior medullary vela, and the epi- 

 thelial lining of the choroid plexuses and tela choroidea inferior. Some small 

 plates of wliite matter, the obex and the ligula^ also enter into the fonuation of 

 the roof. The inferior part of the floor is formed by the medulla and the superior 

 part by the posterior surface of the pons. Some white lines (stride acustic^e), 

 which cross the floor transversely, approximately separate the pontine from the 

 medullary pr)rtion of the floor. 



The valve of Vieussens, or superior medullary velum, is a thin plate of 

 white matter which fills in the angular interval between the superior peduncles of 

 the cerebellum. It is continuous behind with the white matter of the superior 

 vermiform process of the cerebellum. The lingula rests on the posterior part of its 



