LEMUR MONGOZ 65 



cerebclli. At the lateral extremity of the central !j;ra\ matter are several 

 scattered bundles of nerve fibers, the meseiicephalic root 0/ the trigeminal 

 nerve (Tmt), while along the ventral border of the central gray matter 

 are two or three small bundles of libers, the descending fasciculi of the 

 trochlear nerve (N4). 



LEVEL OF THE SUPERIOR COLLICULUS (fIG. 33) 



At the level of the superior collicuhis, prominent tectal structures 

 of the mesencephalon make their appearance. They are the much reduced 

 remnants of the optic lobes which form the most conspicuous elements 

 in the midbrain of the lower classes of vertebrates. Here they retain a 

 certain degree of stratification reminiscent of the optic lobes. Three distinct 

 layers or strata of alternating cell and fiber distribution may be discerned 

 microscopically in the area indicated as stratum griseum superficiale. 



Still other layers may be differentiated in this tectal region of the mid- 

 brain. The optic lobes have been progressively superseded in the impor- 

 tance of their visual function as the cortex of the cerebral hemispheres 

 expanded and became more intimately concerned in those higher associational 

 syntheses necessary for a fuller sensory adjustment to the external world. 

 This supersedence on the part of the cerebral cortex involves not only the 

 sense of vision and the sense of hearing, but quite as much the general 

 body sense in all those various modalities which are essential to the produc- 

 tion of highly organized volitional movements. That all of these specialized 

 forms of sensation, including those which put the animal in touch with 

 elements in its environment more or less remote from its own body, as well 

 as those requiring actual contact with the body surfaces, have in their more 

 primitive states been represented by parts of the brain less highly organized 

 than the cerebral cortex, there can be no doubt. By a slow and gradual process, 

 proceeding step by step from species to species, and onlj- incompletely 



