186 ELEMENTS OF MAMMALIAN ANATOMY. 
median cornu of the lateral ventricle (Fig. 96). The free 
lateral margin of the posterior pillar or crus of the fornix is 
the fimbria, or tenia hippocampi. Adjacent to the median 
line in either hemisphere, a thick lamina of matter, the septum 
lucidum, stretches from the cranial part of the fornix dorsad 
to the callosum. The very narrow cavity formed by the ad- 
herence of the margins of the septum of one hemisphere to 
those of the septum in the other hemisphere is sometimes 
called the fifth ventricle. Between each anterior pillar of 
‘the fornix and the optic thalamus is a cleft, the foramen 
of Monro, leading from the third ventricle, laterad of the 
septum lucidum, into the lateral ventricle. 
The three remaining commissures are known according 
to their location as the anterior, middle, and posterior. 
The anterior commissure (Figs. 95 and 98) perforates the 
corpora striata, extending across the median line imme- 
diately craniad of the anterior pillars of the fornix. It is 
about two millimeters in diameter. The middle commis- 
sure, or massa intermedia, lies between the optic thalami. 
It is sometimes called the soft or gray commissure. It is 
nearly one centimeter in diameter and passes through the 
third ventricle (Fig. 95). The posterior commissure is a 
cord of fibers about a millimeter in diameter connecting the 
caudal portions of the optic thalami. 
The Basal Ganglia.—A semi-independent group of 
nerve cells forming a definite mass is known as a ganglion. 
In the ventral portion of the brain lie three pairs of large 
ganglia, called corpora quadrigenuna, optic thalami, and 
corpora striata. To study them, the entire dorsal surface 
of the brain down to and including the corpus callosum 
should be removed. 
The corpora quadrigemina (Fig. 96) forming the dorsal 
part of the mesencephalon lie craniad of the medulla, and 
consist of an anterior pair and a posterior pair. The for- 
mer are known as the superior colliculi and the latter as 
