a 
. 
y 
THE ROOF OF THE FOURTH VENTRICLE. 473 
ventricle is carried backwards into the cerebellum between the two vela, which 
thus form a peaked and tent-like root for it. 
Relation of the Tract of Gowers to the Superior Medullary Velum.—The 
ascending tract of Gowers has been noticed in connexion with the lateral column of the 
cord (p. 433). The fibres which compose it are carried upwards through the formatio 
Middle cominissure Tenia thalami 
Pineal body 
Quadrigeminal bodies 
| 
| 
CORPUS|CALLOSU, 
eS 
Central lobule 
Choroid plexus 
Foramen of Monro 
Anterior commissure 
Folium 
Sulcus of Monro cacuminis 
. . ¥ A ' i ‘ % Z 7 32 
Lamina cinerea yy) A Re Ge Aa Slaw 5 — Tuber 
Ne valvule 
PITUITARY 
~ Pyramid 
Tuber cinereum 
Corpus maimmillare 
Aqueduct of Sylvius Uvula 
Superior medullary velum ‘ 
Nodule 
Floor of ventricle LV. 
Choroid plexus ventricle IV. 
Fie. 351.—MEsIAL SECTION THROUGH THE CorPuUS CALLOSUM, THE MESENCEPHALON, THE Pons, MEDULLA, 
AND CEREBELLUM, 
Showing the third and fourth ventricles joined by the aqueduct of Sylvius. 
reticularis grisea of the medulla 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 upper end of the pons the tract turns backwards, enters the 
superior medullary velum, and proceeds downwards in it into the cerebellum. 
Roof of the Fourth Ventricle—In its upper part the roof of the fourth 
ventricle is formed by the superior medullary velum as it stretches across between 
the two superior cer ebellar peduncles, and also, to some extent, by the approximation 
of these peduncles themselves as they approach the mesencephalon. 
In its lower part the roof of the ventricle is exceedingly thin and is not all 
formed of nervous matter. The inferior medullary velum enters into its formation, 
and, where this fails, the epithelial lining of the cavity, supported by pia mater, is 
carried downwards towards the lower boundaries of the floor of the ventricle. At 
the lowest part of the calamus scriptorius, and also along each lateral boundary of 
the floor, a thin lamina of white matter is carried for a short distance over the 
epithelial roof. The small lamina at the calamus scriptorius overhangs the opening 
of the central canal, and is termed the obex. The lamina in connexion with the 
lateral boundary of the ventricular floor is more extensive, and is called the ligula 
(Figs. 333 and 335). It begins on the clava and passes upwards over the cuneate 
tubercle to the restiform body. On the outer surface of the restiform body it turns 
outwards so as to bound the lateral recess of the ventricle below, and in some 
cases it may be seen to become continuous around the extremity of the lateral 
recess with the inferior medullary velun. 
A short distance above the calamus scriptorius there is, in the mesial plane, an 
opening in the epithelial and pial roof of the ventricle, by which the cavity of the 
ventricle communicates with the subarachnoid space. This opening is termed the 
foramen of Majendie. There is also an aperture of a similar nature in the epithehal 
and pial roof at the extremity of each lateral recess. 
Two choroid plexuses, or highly vascular infoldings of the pia mater, invaginate 
the lower part of the roof of the fourth ventricle. These are placed one on either 
