Till-: FOURTH VENTRICLE 



795 



thin lamina of white substance, the nntirinr nmliill<iri/ velum, thickened at the sides 

 by the braehia conjunctiva. At its extreme mesencephalic end the anterior med- 

 ullary velum is slightly thickened by a continuation of the white substance of the 

 inferior quadrigeminate bodies, forming the frenulum veli. The inferior portion of 

 the velum is continuous with the while substance of the cerebellum, and is covered 

 by the linguki cerebelli, an extension of the cortical substance of the superior vermis 

 (fur. 582). 



The roof of the middle portion of the fourth ventricle is formed by the cerebellum 

 proper, the vermis and the mesial portions of the hemispheres. The nervous por- 

 tion of the roof terminates with the posterior medullary velum, & thin, narrow band of 

 white substance which is the continuation of the peduncles of the floccular lobes, 

 and which connects them at the mid-line with the nodule of the inferior vermis. 



The roof of the inferior portion of the fourth ventricle is non-nervous. It is the 

 tela chorioidea of the fourth ventricle, a semilunar lamina consisting of the epithelial 

 lining of the ventricle, reinforced by a continuation of the connective tissue of the pia 

 mater and the adjacent portion of the arachnoid. Along the line of its attachment 

 to the surface of the medulla it is thickened, and in sections this portion bears the 

 name ligula (tcenia ventriculi quarti). The thickened portion spanning the tip of 



Fm. 590. DIAGRAM OP THE ROOF AXD LATERAL BOUNDARIES OF THE FOURTH VENTRICLE. 



INFERIOR 



QUADRlGK.it/y.i //; noli y 



FOURTH NERVE 



SUPERIOR MEDl'LLARY 

 VELUM 



BRA CHI I .! 



BRA CHI [ '.V PONTIS 



R KS TIFOR.V BODY 



TV-:/,. i f'/ftniromK.i />/ 



I '< in: Til VKXTKirr.K 



TUBERCULIN! rr.v/-; i rr.v 



CLA t'A 



TUBERCLE OF ROLANDO 



FREKULl'M VELI 

 LATERAL LEMSISCVS 



LINGVLA CEREBELLI 



FOURTH VEKTR1CLE 



INFERIOR MEnn.LARY 



VELUM 

 CHOR/Offl PLEXl'K 



FORA MEN OF MA GEND1E 



OBEX 



the calamus scriptorius is termed the obex. The width of the ventricular cavity is 

 extended outwards from its widest part into the lateral recesses, narrow pockets on 

 each side and around the upper parts of the restiform bodies. In the mid-line of 

 the lower part of the tela chorioidea there is a more or less well-marked opening, 

 the foramen of Magendie (medial aperture of the fourth ventricle) , which is a lymph- 

 channel connecting the cavity of the ventricle with the subarachnoid space. There 

 is a similar opening from each lateral recess (lateral apertures). 



The chorioid plexuses of the fourth ventricle consist of highly vascular, lobular, 

 villus-like processes of the ventricular surface (pia mater) of the tela chorioidea.' 

 They are reddish in the fresh specimen, and the epithelial lining of the ventricle is 

 closely adapted to the uuevennesses of their surfaces. From below they run as two 

 parallel masses on either side of the mid-line, which become united above, and then 

 are separated again into two lateral processes which bend at right angles and pro- 

 ject into the lateral recesses. Portions frequently protrude through the three open- 

 ings of the ventricle into the subarachnoid space. 



The floor of the fourth ventricle (/as.w rJwmboidea) . This is thrown into em- 

 inences and depressions indicative of the internal structures of the brain-stem sub- 

 jacent to it. Its inferior portion is the dorsal surface of the upper portion of the 



