FOURTH VENTRICLE. 603 



tatum. Beneath the corpora quadrigemina, the innermost fibres of each peduncle 

 decussate with each other, so that some fibres from the right half of the cere- 

 bellum are continued to the left half of the cerebrum. 



The inferior peduncles (processus ad medullam) connect the cerebellum with 

 the medulla oblongata. They pass downwards, to the back part of the medulla, 

 and form part of the restiform bodies. Above, the fibres of each process are 

 connected chiefly with the Iamina3, on the upper surface of the cerebellum; and 

 below, they are connected with all three tracts of one half of the medulla, and, 

 through these, with the corresponding half of the cord, excepting the posterior 

 median columns. 



The middle peduncles (processus ad pontem\ the largest of the three, connect 

 together the two hemispheres of the cerebellum, forming their great transverse 

 commissure. They consist of a mass of curved fibres, which arise in the lateral 

 parts of the cerebellum, and pass across to the same points on the opposite side 

 They form the transverse fibres of the pons Varolii. 



FOURTH VENTBICLE. (Fig. 333.) 



The Fourth Ventricle, or ventricle of the cerebellum, is the space between 

 the posterior surface of the medulla oblongata and pons in front, and the cere- 

 bellum behind. It is lozenge-shaped, being contracted above and below, and 

 broadest across its central part. It is bounded laterally by the ptrocessus e cere- 

 bello ad testes above, and by the diverging posterior pyramids and restiform 

 bodies below. 



The roof is arched ; it is formed by the valve of Vieussens and the under 

 surface of the cerebellum, which presents, in this situation, four small eminences 

 or lobules, the nodulus, uvula, and amygdalae. 



The anterior boundary, or floor, is formed by the posterior surface of the 

 medulla oblongata and pons. In the median line is seen the posterior median 

 fissure, which becomes gradually obliterated above, and terminates below in the 

 point of the calamus scriptorius, formed by the convergence of the posterior 

 pyramids. At this point is the orifice of a short canal terminating in a cul-de- 

 sac, the remains of the canal which extends in foetal life through the centre of 

 the cord. On each side of the median fissure are two slightly convex longi- 

 tudinal eminences, the fasciculi teretes ; they extend the entire length of the 

 floor, being indistinct below and of a grayish color, but well marked and whitish 

 above. Each eminence consists of fibres derived from the lateral tract and 

 restiform body, which ascend to the cerebrum. Opposite the crus cerebelli, on 

 the outer side of the fasciculi teretes, is a small eminence of dark gray sub- 

 stance, which presents a bluish tint through the thin stratum covering it; this 

 is called the locus cc&ruleus; and a thin streak of the same color continued up 

 from this on either side of the fasciculi teretes, as far as the top of the ventricle, 

 is called the tsenia violacea. The lower part of the floor of the ventricle is 

 crossed by several white transverse lines, linese transversse ; they emerge from 

 the posterior median fissure ; some enter the crus cerebelli, others enter the 

 roots of origin of the auditory nerve, whilst some pass upwards and outwards 

 on the floor of the ventricle. 



The lining membrane of the fourth ventricle is continuous with that of the 

 third, through the aquaeduct of Sylvius, and its cavity communicates below with 

 the sub-arachnoid space of the brain and cord through an aperture in the layer 

 of pia mater extending between the cerebellum and medulla oblongata. Later- 

 ally, this membrane is reflected outwards a short distance between the cere- 

 bellum and medulla. 



The choroid plexuses of the fourth ventricle are two in number; they are deli- 

 cate vascular fringes, which project into the ventricle on each side, passing from 

 the point of the inferior vermiform process to the outer margin of the resti- 

 form bodies. 



