THE CENTRAL NERl/OUS SYSTEM. 363 



ventricles of the brain, whence the application of the names 

 third and fonrtJi ventricles to the cavities of the 'tween-brain 

 and hindbrain. The lateral ventricles do not grow at the same 

 rate as the walls of the hemispheres, so that they remain com- 

 paratively small. The cavity of each ventricle is further 

 reduced in size by the development on its floor of a large 

 ridge-like thickening, the corpus striatum (Fig. 148, f). 



The dorsal wall or roof of the lateral ventricle joins the thin 

 roof of the third ventricle on each side along an oblique curved 

 line (Fig. 141, i) which follows the cranial or lateral border of 

 the thalamus (Fig. 141, /). Along this line the thin roof of 

 the brain is folded in together with the pia mater to form the 

 choroid plexus of the lateral ventricles (lamina chorioidea 

 epithelialis) (Fig. 148, e). When this is pulled out there is 

 left a fissure, the "great transverse fissure of the cerebrum " 

 (Fig. 141, i). Just dorsad of the groove 

 between the thalami there runs a tract of 

 white fibres known as the fornix (Fig. 

 148, a). The two halves of the fornix 

 separate at the cranial ends of the tha- 

 lami and pass ventrad, forming thus the 

 pillars of the fornix (Fig. 148, b\ Fig. 

 143, v). Dorsad of the cranial end of the FlG M7 . _ THE CORPUS 

 fornix the corpus CallOSum (Fig. 143, /) CALLOSUM. 



passes from one hemisphere to the other. . The dorsal portion of the 



hemispheres has been dis- 

 Caudad of the pillars of the fornix, the sected away, then sliced off, 



lateral ventricles communicate with the 



third ventricle by way of the interven- corpus callosum. , spleni- 



, . ,. //- r -HIT \ um ', b, genu; <r, line marking 



tricular foramen (foramen of Monroe). the me ial edg ' e of the hem f_ 

 The parts of the cerebrum may now spheres; </, line marking the 



lateral boundary of the supra- 

 be taken up in detail. callosal sulcus; laterad of 



The corpus callosum (Fig. 147 ; Fig. . this ! ine . the C <*P Call f t u 1 m 



lies in the substance of the 

 143, /; FigS. 149-152, O] is a broad hemispheres, which have 



transverse band of fibres forming a ^"aTS^ 

 secondary connection between the medial surface. 

 walls of the two hemispheres, dorsad of the roof of the third 

 ventricle. Its outer surface (Fig. 147) is exposed at the 

 bottom of the fissure which separates the hemispheres. On 



