LEMUR MONGOZ 67 



of this partial and graded telcncephalization than in the case of the optic 

 lobes which ultimately give over their dominance in visual function to the 

 occipital area of the cerebral hemispheres. Even within the limited range of 

 the primate order there is still evidence of this slow transference from a 

 highly organized visual region in the midbrain to the more highly developed 

 visual centers of the occipital lobe. The stratification already noted in the 

 superior collicular region which retains, even in this high form of mammal, at 

 least ten of the original fourteen cortical layers of the optic lobe, indicates 

 that tclencephalization has by no means attained its full expression in 

 lemur. It must be inferred, therefore, that some at least of the primordial 

 visual function is vested in these still highly differentiated tectal structures 

 of the midbrain. This supposition is further borne out by the large number of 

 optic fibers which end in the superior colliculi. Some ol these fibers are 

 shown immediately lateral to the stratum griseum superficiale. 



Another feature at this level of the brain stem is the oculomotor nucleus 

 (Noc). This nucleus gives rise to the third cranial nerve whose fibers 

 supply all but two of the muscles which move the eyeball within the orbit. 

 It also innervates the intrinsic muscles of the eye, the constrictor iridis 

 and the muscle of the ciliary body which regulates the convexity of the 

 lens. In addition, the oculomotor nucleus sends fibers to the levator palpe- 

 brae muscle which produces elevation of the upper eyelid. This portion of 

 the midbrain therefore is preeminently related to the visual sense. Not 

 only is this true as to the structures concerned with the actual receipt of 

 visual impressions, but quite as much in the transmission of motor impulses 

 requisite to the muscular adjustments of the eyes in visual fixtaion and in 

 visual pursuit of objects. The fact that this oculomotor nucleus (Noc) 

 shows a striking simplicity in its development indicates a relatively low 

 degree of oculomotor organization. It implies that the lemur's vision is as 

 yet only partially binocular; that in the main the animal is still using its eyes 



