1 88 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 tcciria 

 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. 

 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 



