512 



PONS VAROLII. 



nerve and in front of the grey tnbercle, to reach the middle crus of the 

 cerebelkim. 



The median sejjfiim or raphe, which exists in the medulla oblongata, 

 is prolonged throughout the whole height of the pons in its back part, 

 but becomes indistinct in approaching the front or basilar surface, 

 except towards its upper and lower edge, where the superficial fibres of 

 the pons are manifestly continuous in the median line with these septal 

 fibres. Bundles of white fibres, belonging to the same system, encircle 

 the crura cerebri at their emergence from the npper border of the 

 pons. 



The greij matter consists of small round and fusiform cells and forms 

 a network among the bundles of fibres. 



The fourth ventricle. — The space left between the medulla ob- 

 longata in front, and the cerebellum behind (figs. 358, and 362, n ; 

 384, V 4), is named the fourth ventricle, or ventricle of the cerehellum/ 



The cavity of the ventricle is of a flat rhomboidal shape, being con- 

 tracted above and below, and widest across its middle part. It is 

 bounded laterally by the superior peduncles (fig. 358, 5), and by the 

 line of union of the medulla oblongata and the cerebellum. Behind, it is 

 covered in above by the valve of Vieussens, Avhich extends across between 

 the superior peduncles of the cerebellum, and below by part of the 

 inferior vermiform process of the cerebellum which projects into it. 

 The upper end of the ventricle is continuous with the Sylvian aqueduct 

 or passage (iter) leading up to the third ventricle. 



Fig. 358. 



Fig. 358. — View of the 

 Flook of thk Fourth Ven- 

 tricle WITH THE Posterior 

 Surface of the Medulla 

 Oblongata and neigh- 

 bouring PARTS (from Sappey 

 after Hii'schfeld and Le- 

 veille). 



On the left side tte tliree 

 cerebellar peduncles have been 

 cut short ; on the right side 

 the white substance of the 

 cerebellum has been preserved 

 in connection with the supe- 

 rior and inferior peduncles, 

 while the middle one has been 

 cut short. 



1, median groove of the 

 fourth ventricle with the fas- 

 ciculi teretes, one on each 

 side ; 2, the same groove at 

 the place where the white 

 strite of the auditory nerve 

 emerge from it to cross the 

 floor of the ventricle ; 3, inferior peduncle or restlform body ; 4, posterior pyramid ; 

 above this the calamus scriptorius ; 5, superior peduncle or processus a cerebello ad 

 cerebrum ; on the right side the dissection shows the superior and inferior peduncles 

 crossing each other as they pass into the white centre of the cerebellum ; 6, fillet to the 

 side of the crura cerebri ; 7, lateral grooves of the crura cereljri ; 8, corpora quadri- 

 gemina. 



The anterior surface or floor of the fourth ventricle is fonned by the 

 back of the medulla oblongata and pons Varolii. It is shaped like a 



