200 ELEMENTS OF MAMMALIAN ANATOMY. 



the lateral walls of the third ventricle, across which they 

 meet, forming the massa intermedia, or middle com- 

 missure. 



On the dorsal aspect of each thalamus, near the median 

 line, is a longitudinal band of white fibers called the 

 habena, or t&nia thalami, which at its caudal limit is 

 united to its fellow by the commissura habenae. 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. 94), 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 projection of the thalamus. 

 The internal capsule, well shown in a transverse section, 

 is composed mainly of the fibers descending from the 

 cells of the parietal region of the cortex. 



The pineal gland, or pineal body (Fig. 92), is a conical 

 projection about a half centimeter long, from the caudal 

 part of the dorsal surface of the thalamus. It is a ves- 

 tigial 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 are the most cephalic of the basal 

 ganglia and are somewhat pear-shaped, the larger ends 

 being cephalad and nearer the median line than the 

 caudal portions, which curve laterad around the optic 

 thalami (Fig. 93). They form a part of the floor of the 

 lateral ventricles, and are pierced by the anterior com- 

 missure (Fig. 95), a small cord of white fibers. 



