718 



THE yKRVOCS SYSTEM 



upper surface. The fibres of the fourth nerve decussate within its substance at its 

 upper part. 



The inferior medullary vela are thin crescentie transhicent bands of Avhite 

 matter which extend from the noduk^ to the flocculus on each side; hence the}' 

 have been called commissurae ad flocculos. Each velum presents a concave 

 edge, which is apparently free, but which is in reality continued into the epithe- 

 lium, which covers the choroid plexuses. It is attached to the cerebellum by its 

 convex edge. 



The choroid plexuses of the fourth ventricle are convoluted vascular folds of 

 the i)ia mater, wliich project into the cavity of the fourth ventricle, but are excluded 

 horn tlie ventricular cavity by a layer of epithelium. The}- run at first forwards 

 close together on each side of the middle line, and then turn outwards along the 

 edges of the inferior medullary vela to reach the lateral recesses of the ventricle, 

 where they become continuous with tlie general pia mater. 



Fig. 427. — Eight Half of the Encephalic Peduncle as seen from the Inside of a 

 Median Section. (Allen Thomson, after Keichert. ) 



MIDDLE COMMISSURE 



OPTIC THALAMUS 



PIXEAL BODY 



CORPORA QUADRIGEMIXA 



DECEIVE rT*.-r*>^ 1 



CR US CEREBRI ^ ?^^^"^*^ 



STRIA PISE A LIS 

 THIRD VEXTRICLE 



AXT. PILLAR OF FORNIX 

 ASTERIOR COilMISSURK 



' "m- LAMIXA CIXEREA 



TVBER CIXERECM 

 REI'ESSUS I.VFl'X- 

 DIBCLI 



PITUITARY BODY 



AQUEDUCT OF SYL VIUS 



POXS 



FO I R TH I 'EXTRICLE 



IXFERIOR MED ULLA R Y VEL UM 



MEDULLA 



1, culmcu ; 2, posterior crescentie lobe ; 3, posterior inferior lobe ; 4, slender lobe ; 5, bi ventral 



lobe ; 6, amygdala. 



The tela choroidea inferior is a layer of pia mater which closes in the lower 

 and back part of the foiu-tli ventricle. It extends from the choroid plexuses to the 

 ligulffiand obex, where it is continued into the pia mater which covers the posterior 

 surface of the lower part of the medulla. As already mentioned in the description 

 of the meninges, it presents three openings or deficiencies: one (foramen of 

 Majendie) in the middle line immediately aliove the oliex, and two (foramina of 

 Key and Retzius ) at the lateral recesses. 



The obex is a thin, somewhat triangular plate of white matter, occupying the 

 angular interval between the two diverging clava3. It is not infrequently al)sent. 



The ligulae are thin narrow strips of Avhite matter, which project from the inner 

 margins of the clavae. They run at first upwards and forwards, and then turn 

 outwards around the restiform bodies. They may sometimes be traced as far as 

 the flocculi, when they become continuous with the inferior medullary vela. Their 

 inner, apparently free, edges are continuous with the epithelial roof of the fourth 

 ventricle. 



