838 MAN 



OiK' olIuT l(.';iturc' which at once attracts attt'iitioii 111 this (general sur\cy 

 of the exposed nuclear material is the coinparatixc decrease m the size and 

 importance of the reticuhir formation, the matrix out of which has de\ eloped 

 the great majority of the nuclear masses belonging to the gra\ matter of the 

 brain stem. The reconstruction m general does not give the impression of 

 massivcncss which might be expected in this the largest brain stem of the 

 series. The nuclear accumulations, although increased relatnely m size, 

 dillt-r materially from those which have been observed in the lower members 

 of the series in an increasing sharpness of definition, a greater degree of deli- 

 cacy and clearness ol outline, w hich serves more sharplx to dillerentiate them 

 irom those ol the lower primates than does any material increasein bulkorsize. 

 The fundamental structures of the brain stem, the reticular formation, the 

 nuch'i ot the cranial nerves and the specialized masses of gray matter in the 

 region of the mesencephalon, particularly the nucleus ruber and tiresubstantia 

 nigra, do not present any material alteration in the a|)pearance of these striic- 

 tures as they have been observed in the lower orders in this series. 



The organized masses w hich ha\ e materially changed in size, complexity 

 and in contour are those connected with the iunctions which in man have 

 achieved a greater degree of integration and coordination than that attained 

 by any of the lower members of the primate group. This |)articularly applies 

 to the pontile nucleus which develops, pan jjassu, with the expansion of the 

 cerebral hemispheres and organization of synergy, the contribution of the 

 cerebellum to animal motion. The olixary nucleus which must be considered 

 as a comparative newcomer 111 the organization of tlu' lirain stem, as it 

 presents material comple\it\- only in the upper primates, shows a marked 

 advance m man oxer that which has bt'cii observed in the other members of 

 this group. Although its liinction is not tiill\ understootl, its de\ clopnuMit has 

 been steady, dehnite and continued. The conclusion heri' acKanced ma\- not 

 yet be entirely supported 1)\ delinite e\ idcnce, iu\ trtlulcss the increase in 



