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. 
Fic. 97. CROSS-SECTION OF THE BRAIN IN THE PLANE ¥ IN FIG, 95. 
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; cx, choroid plexus of lateral ventricle; cx’, 
choroid plexus of third ventricle; cxt, gray cortex; ca, ependyma 
lining the ventricles; f, median longitudinal fissure; h, tenia 
thalami; mc, middle commissure, or massa intermedia; n, fibers 
of the optic tract as they enter the lateral geniculate body s; 
o, fornix; 7, caudal portion of nucleus lenticularis; op, optic 
thalamus ; oc, optic tract as it leaves the chiasm; sl, septum lucidum; 
vn, part of third ventricle dorsad to the commissure; wv, 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. 
