INTERNAL STRUCTURE OF THE BRAIN STEM 1035 



In similar iiianiuT, tlu- c'\])ansi<)n ol \isual luiU'tiDii wliifli has lakrii 

 plact' in the ])r()<irc'ssivc augmentation of tlic occipital lobe makes its con- 

 tribution to the organization and acKaneement ol neokinesis. A large pro- 

 portion of all skilled motor ptatormanees in man depend primaril\ upon 

 vision for their aec]uisition. They are ec|uall\ ck'j)t'ndent upon \ isual integrity 

 lor their execution. This apjjlies ])artieularly to the minute and |)recise skilled 

 acts of the hand mxoKfd in writing, m drawing, m jjamting, in sculpture. 

 It must l)e apparent, t hen-lore, t hat t he ])iirposi' ol pro\ iding \ ision and hear- 

 ing with tlu'ir ultimati- opportunities lor expansion in the cerebral cortex is 

 especially concerned in the inlliu'iui' which they exert u])on the de\ (.■lopmeiit 

 and maintenance ol neokinetic acti\ities. 



In passing Irom the lower primates to man, the progressn (.• de\ c'lop- 

 meiit ol the oculomotor decussation is so striking as to lea\'c no doubt con- 

 cerning Its e\()lutional signilicance. As inter[jreted in this discussion, it makes 

 pro\ision lor the most complete- con|ugate association of the eyes. The 

 absence of such mti'rmiclear connection would scarcely permit of that 

 intimate cooperation between the nioxc-nu'iits ol the excballs essential to 

 liolding the \isual axes in ])aralK'l or producing those degrees of coinergence 

 necessary to stereoscopic \ision. In other words, without this connection the 

 appreciation of objects and their spatial relations, more particularl\ in the 

 near ranges of \ision, would be incomplete and the more exact a])plication 

 ol liner manual movements would thus be gri'atly curtailed, i lowessentialsucii 

 stereoscopic \ ision is to tlu- skilled acts of chirography, painting and sculp- 

 ture scarcc'lv needs mow than nuntion to establish its claim, l-rom this 

 \iewpoint It IS again e\ ident that the specific ]:)urp()ses of neokinesis ha\e 

 been ser\t(l by the de\ clopnunt of t he mternuclear connections of tiie oculo- 

 motor nerves. By mensuration the disparity between tlu' lemur and man, 

 in the oculomotor decussation, is the most pronounced of all figures ciuoted. 

 I^eniur has a re|)resentat ion m its longitudinal coellicient of iC) ])er cent, while 



