THE NERVOUS SYSTEM. 



187 



inferior colliculi. The anterior pair lie nearer to the midline 

 than the posterior pair, which are slightly separated by a de- 

 pression occupied by the middle portion of the central lobe 

 of the cerebellum. The posterior pair are united by a white 

 commissure. The posterior commissure of the brain unites 

 the cranial portions of the anterior pair (Fig. 95). Its 

 cut end may be seen ventrad to the base of the pineal gland. 



FIG. 97. CROSS-SECTION OF THE BRAIN IN THE PLANE x IN FIG. 95. 

 The plane is just caudad of the optic chiasm. 



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

 anterior pillars of f ornix ; ex, choroid plexus of lateral ventricle ; ex', 

 choroid plexus of third ventricle; cxt, gray cortex; ea, ependyma 

 lining the ventricles; f, median longitudinal fissure; h, tsenia 

 thalami ; wr,, middle commissure, or massa intermedia; n, fibers 

 of the optic tract as they enter the lateral geniculate body s; 

 o, f ornix ; r, caudal portion of nucleus lenticularis ; op, optic 

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

 vn, part of third ventricle dorsad to the commissure; v, third 

 ventricle; t. fibers of optic tract. 



Laterally each pair of the corpora quadrigemina is pro- 

 longed into two white bands, the anterior and posterior 

 brachia. The latter are about a half centimeter long, and 

 pass forward beneath a pisiform ganglion, the corpus geni- 

 culatum internum or mediale. The anterior brachia pass 

 from the cranial end of the posterior pair laterad beneath 

 the caudal projections of the optic thalami, where they join 

 the optic tracts. 



