578 THE NERVOUS SYSTEM. 



middle, the inferior, and the superior (Fig. 519, p. 585). The fibres of which 

 they are composed all enter or emerge from the white medullary centre of the 

 cerebellum. 



The middle peduncle or brachium pontis is much the largest of the three, 

 and has already been described on pp. 565 and 566. It is formed by the 

 transverse fibres of the pons, and it enters the cerebellar hemisphere on the 

 lateral aspect of the other two peduncles. The lips of the anterior part of the 

 horizontal fissure are separated widely from each other to give it admission 

 (Fig. 510). Within the cerebellar hemisphere its fibres are distributed in two 

 great bundles. Of these, one, composed of the superior transverse fibres of the pons, 

 radiates out in the inferior part of the hemisphere ; whilst the other, consisting of 

 the inferior transverse fibres of the pons, spreads out in the superior part of the 

 hemisphere. 



The inferior peduncle is simply the restiform body of the medulla oblongata. 

 After leaving the medulla oblongata it ascends for a short distance on the dorsal 

 surface of the pons and then turns sharply backwards, to enter the cerebellum 

 between the other two peduncles. 



The superior peduncle or brachium conjunctivum, as it issues from the cerebellum, 

 lies close to the medial side of the middle peduncle (Fig. 512). Its further course 

 upwards on the dorsum of the pons to the inferior quadrigeminal body has been 

 previously described (pp. 548 and 569). 



Connexions established by the Peduncular Fibres. The fibres of the brachium 

 pontis represent the second stage of the connexion between the cerebral hemisphere 

 of one side and the opposite cerebellar hemisphere. The connexions which they establish 

 in the pons are described on p. 566. 



The restiform body is also composed of afferent fibres (see p. 563) ; only the more 

 important connexions which these establish in the cerebellum can be touched on here. 

 The principal afferent strand is the fasciculus spinocerebellaris [posterior]. The fibres of 

 this strand end in the cortex of the superior vermis on both sides of the median plane, but 

 chiefly on the opposite side. The olivo-cerebellar tract (fasciculus olivocerebellaris) are also 

 afferent. It appears that they end in connexion with cells in the cortex of both the 

 vermis and hemisphere, and also with cells in the nucleus dentatus. The numerous 

 external arcuate fibres which enter the restiform body establish connexions with cells in 

 the cortex of the hemisphere and of the vermis. 



The brachium conjunctivum is an efferent tract : its fibres come from the cells of the 

 nucleus dentatus, and pass to the red nucleus and thalamus of the opposite side. 

 According to Ramon y Cajal collateral branches springing from these fibres descend to 

 the motor nuclei in the medulla oblongata and spinal medulla. 



There is, however, a bundle of fibres passing downwards alongside the brachium 

 conjunctivum from the tegmentuin of the mesencephalon and possibly from the 

 thalamus : these fibres cross in the mid-brain and pass inferiorly to the cerebellum, 

 in contact with the lateral margin of (or intermingled with) the fibres of the 

 brachium. They probably convey to the cerebellum fibres from the visual centres 

 of the opposite side. 



The fasciculus anterolateralis superficialis of the spinal medulla (O.T. Gowers' tract), 

 for which the better name fasciculus spinocerebellaris anterior is now in common use, also 

 enters the cerebellum alongside the emerging brachium conjunctivum. It has been 

 noticed in connexion with the lateral funiculus of the spinal medulla (p. 537). The 

 fibres which compose it are carried upwards through the formatio reticularis grisea of 

 the medulla oblongata and the corresponding part of the tegmental portion of the pons. 

 In this part of its course the fibres are scattered and do not form a compact strand. 

 Reaching the superior end of the pons the tract turns backwards across the brachium 

 conjunctivum, enters the anterior medullary velum, and proceeds downwards in it into 

 the cerebellum. 



Roof of the Fourth Ventricle. In its superior part the roof of the fourth 

 ventricle is formed by the anterior medullary velum as it stretches across between 

 the two brachia conjunctiva, and also, to some extent, by these brachia themselves 

 as they approach the mesencephalon. 



