188 ELEMENTS OF MAMMALIAN ANATOMY. 
Homologues of the corpora quadrigemina exist in all 
vertebrates. In these bodies originate partly the optic 
nerves, and therefore their size is in proportion to the ani- 
mal’s power of sight. In the mole, which has little use for 
eyes, the anterior pair is rudimentary. 
The optic thalami forming the sides of the diencephalon 
or tween brain are the largest pair of basal ganglia, and 
lie craniad of the corpora quadrigemina and form the 
lateral walls of the third ventricle, across which they meet. 
forming the massa intermedia, or middle commissure. 
On the dorsal aspect of each thalamus, near the median 
line, is a longitudinal band of white fibers called the tena 
thalami, which at its caudal limit is united to its fellow by 
the commissura habene. The thalamus is composed largely 
of gray matter, but there are two important bundles of 
fibers, known as the optic tract and internal capsule (Fig. 
97), appearing on its lateral surface. The optic tract, of 
which the optic nerve is a continuation, arises by two roots, 
the larger of which comes from the cells forming the 
lateral geniculate body, which is the lateral and caudal pro- 
jection of the thalamus. The internal capsule, well shown 
in a transverse section, is composed of the fibers forming a 
communication between the cells of the cortex and those in 
the lower brain centres and spinal cord. 
The pineal gland, or pineal body (Fig. 95), is a conical 
projection about a half centimeter long, from the caudal 
part of the dorsal surface of the thalamus. It is a vestigial 
structure which in some of the lower vertebrates in early 
geological time functioned as a third eye. In Hatteria, 
a New Zealand lizard about a foot long, the eye is present, 
projecting slightly through a foramen in the parietal bone. 
Traces of this third eye with a lens have also been noticed 
in the embryo of the viper and some of the lizards. 
The corpora striata lie deep in the telencephalon. They 
