THE NERVOUS SYSTEM 221 



Homologues of the corpora quadrigemina exist in all verte- 

 brates. In these bodies originate partly the optig nerves, and 

 therefore their size is in proportion to the animal's power of 

 sight. In the mole, which has little use for eyes, the anterior 

 pair is rudimentary. 



The IhaJami 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 tcEuia ihalami, 

 which at its caudal limit is united to its fellow by the commis- 

 sure habenge. 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. io8), appearing on its lateral sur- 

 face. 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 cau- 

 dal projection 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 centers and spinal cord. 



The pineal gland, or pineal body (Fig. io6), is a conical projec- 

 tion 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 are 

 the most anterior of the basal ganglia and are somewhat pear- 

 shaped, the larger ends being craniad and nearer the median 



