THE FOURTH VENTRICLE. 47 



The pons consists ventrally of transverse cerebellar fibres, between which 

 the longitudinal fibres prolonged upwards from the medulla oblongata pass ; 

 together with a large intermixture of grey matter. The superficial fibres on the 

 ventral surface (fig. 30, p) are transverse in their general direction, but while the 

 middle fibres are exactly transverse, the lower set ascend slightly, and the superior 

 fibres (fig. 30, i), which are the most curved, descend obliquely to reach the eras 

 cerebelli on each side ; some of the upper fibres cross obliquely the middle and 

 lower ones, so as to conceal them at the sides. When the superficial transverse 

 fibres are removed, the prolonged fibres of the pyramids come into view (fig. 36) ; 

 these, as they ascend through the pons, are separated into smaller bundles, 

 intersected by other transverse white fibres, which, with those upon the surface, 

 are all continued into the middle peduncle of the cerebellum. 



At the lower part of the pons, dorsal to the fibres from the pyramids, is a 

 special set of transverse fibres (fig. 38, /), named the trapezium (corpus trapez- 

 oides) so called because in most mammals, in which the inferiorly situated fibres of 

 the pons are less developed and the pyramids are small, these transverse fibres 

 partially appear on the surface in an area of a somewhat four-sided shape. Laterally 

 they curve round a collection of grey matter, called the superior olivary nucleus 

 (fig. 38, 0.s.), and probably many of them are connected with its cells. They then 

 course lateralwards, across the bundles of the facial nerve-roots (F77), and ventral 

 to the upward prolongation of the substantia gelatinosa of the tubercle of Rolando 

 and the bundles of fibres belonging to the ascending root of the fifth nerve (#.F), 

 and appear to become connected with the accessory auditory nucleus (p. 55) and 

 with the ventral root of the auditory nerve, of which, according to Flechsig, the 

 trapezium forms a cerebral commissure. 



The fourth ventricle. The external characters of the medulla oblongata and 

 pons may be completed by a description of those parts which enter into the 

 boundaries of the fourth ventricle. This is the space into which the central canal 

 of the cord, after becoming somewhat enlarged and cleft-like, opens out superiorly 

 (fig. 39). The opening-out seems as if effected by the divergence of the funiculi 

 graciles et cuneati on either side at an acute angle. These funiculi, which form the 

 lateral boundaries of the lower part of the ventricle, bend round laterally at about 

 the middle (in length) of the medulla oblongata, so that the ventricle, which is at 

 first narrow, rapidly broadens. Opposite the middle peduncle of the cerebellum it 

 has attained its greatest width. From this point its upper part again narrows, con- 

 verging gradually above to be continued into the comparatively narrow Sylvian 

 aqueduct. The ventricle is therefore irregularly lozenge- or diamond-shaped, and 

 is sometimes named fossa rhomboidalis. The pointed lower end of the ventricle 

 has the shape of a writing pen, and is termed the calamus scriptorius (fig. 39, c.s.}. 

 At its widest part the fourth ventricle is continued for a short distance on either 

 side between the cerebellum and bulb where these come in contact, in the form 

 of the pointed lateral recess (Lr.). The lateral boundaries of the ventricle 

 are, in its lower or medullary part, the clavae of the funiculi graciles, the funiculi 

 cuneati, and the restiform bodies ; in its upper half the superior peduncles of the 

 cerebellum (fig. 39, s.c.p.}. These pass gradually to the roof of the ventricle as 

 they extend upwards. They are at first separated from one another by a tolerably 

 wide interval which, however, gradually narrows near the end of the ventricle, 

 the two crura of opposite sides there approaching one another, and their margins 

 coming in contact. Roof of the ventricle. The triangular interval between 

 the two crura is bridged over by a lamina of white matter marked across with 

 grey streaks. This is the superior medullary velum or valve of Vieussens (fig. 

 39, s.m.v.), and, with the crura, forms the roof or dorsal boundary of the upper 

 part of the fourth ventricle. The white substance of which it is mainly composed 



