806 



< I:\THM. 



Fig. 331. 



Tin: 



thick nnolcns of groy substance that crosses the longitudinal fibres of tho 

 encephalic isthmus in passing into tho hemispheres ; these fibres appear in 

 several points of this nucleus as very distinct white streaks. 



This deep nucleus, which is in- 

 termediate to tho superior extremity 

 of tho isthmus and tho principal 

 mass of tho hemisphere, comprises 

 the entire thickness of tho floor of 

 tho lateral ventricle, and projects 

 outwards, beneath the inferior face 

 of tho hemisphere, between the two 

 roots of tho olfactory lobe, where 

 it constitutes tho extra-ventricular 

 nucleus of the corpus striatum : so 

 named in contradistinction to tho 

 oblong eminence in the interior 

 of the ventricle, which is often de- 

 signated the intra-ventricular nucleus 

 of the corpus striatum. 



7. The Velum Interpositum and 

 Choi-old Plexus. (Fig. 331, G.) 



ANTKHIOR PORTION OF THE LATERAL vENTRi- The velum interposition (velum 



CLES OF THE DOG, EXPOSED BY HEM OVAL OF vasculosum, tela choroidea) is a vas- 



THE ROOF. ciilar expansion dependent from the 



1, Corpus callosum ; 2, Anterior part of the p i a mater, which penetrates the 



corpus callosum, turned forward after destruc- i i ^ *.,,., fi collro an A 



corpu 

 tion 



US P ^ 1 - brain by the transverse fissure, and 



LIUU of the septum lucidum, to show the formx. . . J ,!,, 



3, 3; 4, 4, Hippocampi; 5, 5, Taenia semi- insinuates itself between the tha- 



circularis ; 6, 6, Choroid plexus ; 7, 7, Corpora lamus opticus and the convolution 



.-ma tu. of the cornu Ammonis. The velum, 



on arriving beneath tho teenia hip- 



pocampus, terminates in tho choroid plexus : a red, granular-looking cord, 

 which is suspended by its antero-external border, and projects into the 

 interior of the Literal ventricle. 



The choroid plexuses of the brain extend from the anterior extremity of 

 the corpus striatum to the bottom of the ccecum in tho mastoid eminence 

 or lobule. In the anterior region of the ventricle, they occupy the oblique 

 sulcus which traverses that part, to the inner side of the corpus striatum. 

 In the posterior region, they float in front of the cornu Ammonis. Their 

 anterior or internal extremity, more voluminous than tho external, always 

 forms a small appendage which remains quite free. They are united to 

 each other, near this extremity, by an intermediate cord, which traverses 

 the foramen of Monro in passing beneath the fornix. 



Like the velum interpositum, the choroid plexuses are formed by a net- 

 work of arteries and veins. They are often incrusted in calcareous matter, 

 and may be the seat of more or less voluminous cysts. 



The veins proceeding from this vascular apparatus are very voluminous, 

 and by their union form tho great vena Galeni, which bends round tho 

 splenium of the corpus callosum to reach the interlobular fissure, and pro- 

 ceeds to the sinus of the falx cerebri. 



