THE CENTRAL NERVOUS SYSTEM. 363 
ventricles of the brain, whence the application of the names 
third and fourth ventricles to the cavities of the ’tween-brain 
and hindbrain. ‘lhe 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) (Fig. 148, ¢). 
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, 4; Fig. 
143, v). Dorsad of the cranial end of the 
fornix the corpus callosum (Fig. 143, p) 
passes from one hemisphere to the other. 
Caudad of the pillars of the fornix, the 
lateral ventricles communicate with the 
third ventricle by way of the interven- 
tricular foramen (foramen of Monroe). 
The parts of the cerebrum may now 
be taken up in detail. 
The corpus callosum (Fig. 147; Fig. 
143, ~; Figs. 149-152, a) is a broad 
transverse band of fibres forming a 
secondary connection between the medial 
726 
Fic. 147. — THE Corpus 
CALLOSUM. 
The dorsal portion of the 
hemispheres has been dis- 
sected away, then sliced off, 
showing the plate of trans- 
verse fibres forming the 
corpus callosum. a, spleni- 
um; 4, genu; ¢, line marking 
the medial edge of the hemi- 
spheres; d, line marking the 
lateral boundary of the supra- 
callosal sulcus; laterad of 
this line the corpus callosum 
lies in the substance of the 
hemispheres, which have 
been dissected away; ¢, line 
marking medial limit of cut 
surface. 
walls of the two hemispheres, dorsad of the roof of the third 
ventricle. 
bottom of the fissure which separates the hemispheres. 
Its outer surface (Fig. 147) is exposed at the 
On 
