PAPIO CVNOCEPHALUS 329 



to the regulation of certain automatic associated movements. The central 

 gray matter (Cen), considerably enlarged as compared with the lower 

 levels, surrounds the caudal extremity of the Sylvian aqueduct. At its 

 lateral extremity are several scattered bundles of nerve libers, the mesen- 

 cephalic root of the trigeminal nerve, while mesial to this root along the 

 ventral border of the central gray matter are two or more bundles of libers, 

 the descending fasciculi of the trochlear nerve (, N4 ). Upon its extreme ventral 

 aspect the central gray matter is bounded by fibers forming the fasciculus 

 longitudinalis posterior (PL) and the fasciculus predorsalis (PD). 



LEVEL OF THE SUPERIOR COLLICULUS (fIG. I 54) 



At this level conspicuous mesencephalic structures make their appear- 

 ance. Chief among these are the much reduced remnants of the optic lobes oi 

 the lower vertebrates. These lobes, formerly serving as the end stations ot 

 visual sensibility, still retain a degree of stratification reminiscent of their 

 condition in the lower forms. At least three distinct strata of alternating cells 

 and fibers may be discerned microscopically. During the process in which the 

 optic lobes have progressively delegated their visual function to the occipital 

 areas in the cerebral cortex, a similar supersedence by the cerebral hemi- 

 spheres has involved the sense of hearing as well as those modalities of 

 general body sensibility essential to the production of the most highly 

 organized volitional movements. It is still evident even in the primate order 

 that a slow transference from the visual region of the midbrain to the higher 

 visual centers of the occipital lobe is in process. 



Another element in this level is the oculomotor nucleus (Noc) which 

 gives rise to the third cranial nerve whose fibers supply all but two ot 

 the muscles moving the eyeball. The fact that this oculomotor nucleus 

 shows a relatively simple degree of development, particularly in its inter- 

 nuclear fibers, implies a relatively low degree ofoculomotor organization. These 



