THB CENTRAL NERVOUS SYSTEM. Z^l 



ventricles of the brain, whence the application of the names 

 third and fourtJi 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,/). 



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, 1) 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) (Pig. 148, c). When this is pulled out there is 

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

 (Fig. 141, 1). 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\ Pig. 



14^,7'). Dorsad of the cranial end of the tt, , . , .p r- 

 ^-" ' Fig. 147. — The Corpus 



fornix the corpus callosum (Pig. 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 shced off, 

 lateral ventricles communicate with the t^;^^^^'^^ ^Y"" "^ '"""T 



verse fibres fornung tlie 



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



,.,/. /-- r-RT \ \vn\\ b, genu; f, line marking 



tncular foramen (foramen of Monro). the medial edge of the hemi- 



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



, . 1 •! lateral boundary of the supra- 



be taken up m detail. callosal sulcus; laterad of 



The corpus callosum (Fig. 147 ; Fig. j!"^ '.'"^ /he corpus callosum 

 •^ \ o -r/ ' i,gg jj^ the substance of the 



143, p\ Figs. 149-152, -a) is a broad hemispheres, which have 



transverse band of fibres forming a been dissected away; .line 



o markmg medial hmit of cut 



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 



