696 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



Fig. 331. 



thick nucleus of grey substance that crosses the longitudinal fibres of the 

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

 several points of this nucleus as very distinct white streaks. 



This deep nucleus, which is in- 

 termediate to the superior extremity 

 of the isthmus and the principal 

 mass of the hemisphere, comprises 

 the entire thickness of the floor of 

 the lateral ventricle, and projects 

 outwards, beneath the inferior face 

 of the hemisphere, between the two 

 roots of the olfactory lobe, where 

 it constitutes the extra-ventricular 

 nucleus of the corpus striatum : so 

 named in contradistinction to the 

 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 

 Choroid Plexus. (Fig. 331, 6.) 



ANTERIOR PORTION OF THE LATERAL vENTRi- The velum interpositum (velum 

 CLES OF THE DOG, EXPOSED BY REM OVAL OF vasculosum, tela choroideu) is a vas- 

 THE ROOF. cular expansion dependent from the 



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

 corpus callosum, turned forward after destruc- brain b the transverse fissure, and 

 turn of the septum lucidum, to show the fornix, . . . M* -> > 

 3, 3; 4, 4, Hippocampi; 5, 5, T^nia semi' insinuates itself between the tha- 

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

 striata. of the cornu Ammonis. The velum, 



on arriving beneath the taenia hip- 

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

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

 interior of the lateral ventricle. 



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

 the corpus striatum to the bottom of the ccecum in the 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 the 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 the great vena Galeni, which bends round the 

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

 ceeds to the sinus of the falx cerebri. 



