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NEUROLOGY 





interval, together with the lower portions of the choroidal fissures, is sometimes 

 spoken of as the transverse fissure of the brain. At its base the two layers of the 

 velum separate from each other, and are continuous with the pia mater investing 

 the brain in this region. Its lateral margins are modified to form the highly vas- 

 cular choroid plexuses of the lateral ventricles. It is supplied by the anterior and 

 posterior choroidal arteries already described. The veins of the tela chorioidea are 

 named the internal cerebral veins (venae Galeni); they are two in number, and run 

 backward between its layers, each being formed at the interventricular foramen by 

 the union of the terminal vein with the choroidal vein. The internal cerebral 

 veins unite posteriorly in a single trunk, the great cerebral vein (vena magna Galeni), 

 which passes backward beneath the splenium and ends in the straight sinus. 





Fio. 750. Tela chorioidea of the third ventricle, and the choroid plexus of the left lateral ventricle, exposed 



from above. 



Structure of the Cerebral Hemispheres. The cerebral hemispheres are composed 

 of gray and white substance: the former covers their surface, and is termed the 

 cortex; the latter occupies the interior of the hemispheres. 



The white substance consists of medullated fibers, varying in size, and arranged 

 in bundles separated by neuroglia. They may be divided, according to their 

 course and connections, into three distinct systems. (1) Projection fibers connect 

 the hemisphere with the lower parts of the brain and with the medulla spinalis. 

 (2) Transverse or commissural fibers unite the two hemispheres. (3) Association 

 fibers connect different structures in the same hemisphere; these are, in many 



