THE NERVOUS SYSTEM. 



prolonged into two white bands, the anterior and pos- 

 terior brachia. The latter are about a half centimeter 

 long, and pass forward beneath a pisiform ganglion, the 

 corpus geniculatum internum or mediate. The anterior 

 brachia pass from the cephalic end of the nates laterad 

 beneath the caudal projections of the optic thalami, 

 where they join the optic tracts. 



FIG. 94. CROSS-SECTION OF THE BRAIN IN THE PLANE x IN FIG. 92 

 The plane is just caudad of the optic chiasm. 



1 and 2, First and second or lateral ventricles; cr, corpus callosum; cm, 

 anterior pillars of fornix; ex, choroid plexus of lateral ventricle; ex', 

 choroid plexus of third ventricle; cxt, gray cortex; ca, ependyma or 

 endyma lining the ventricles; /, median longitudinal fissure; h, 

 habena, or taenia thalami; me, middle commissure, or massa inter- 

 media; n, fibers of the optic tract as they enter the external genic- 

 ulate body s; o, fornix ; r, caudal portion of nucleus lenticularis ; 

 op, optic thalamus ; oc, optic tract as it leaves the chiasm ; si, septum 

 lucidum; im, part of third ventricle dorsad to the commissure; v, 

 third ventricle ; /, fibers of optic tract. 



Homologues of the corpora quadrigemina exist in all 

 vertebrates. In them originate largely the optic nerves, 

 and therefore their size is in proportion to the animal's 

 power of sight. In the mole, which has little use for 

 eyes, the anterior pair is rudimentary. 



The optic thalami form the largest pair of basal ganglia, 

 and lie cephalad of the corpora quadrigemina and form 



