INTERNAL STRUCTURE OF THE BRAIN STEM dh 



and pass tlirouyli \hv ccic-hral pe'diHK'k- and jjdiis into the nutlulla oblongata. 

 Each spc'C'iali/c'd aixvi ol the motor forlcx, siuli, lor iwainplc. as the I(.'<j; area, 

 contributes axons to the |j\ianiidal tract. Accordinfi as such motor area ol 

 the neopalluim is e\ti'nsi\ c'l\ ii'picsented, tlu' numljtr ol libers arising in it 

 is c()rres])ondmgly large or small. 1 he progressi\t' expansion ol the pri'ct-ntral 

 area m all ol the primates is one ol the most noteworthy lealures in the 

 de\ flopnu-nt ol tlu- brain. The motor area itsell is acti\ated and giiidt-d by 

 neural mllueiiees Irom man\ other regions ol the neopallium. It is, as it were, 

 merely tlu' mouthpiece ol the cerebral hemispheres which gi\cs linal expres- 

 sion to the neural combinations created withm the entire cerebral cortex. 

 Idlling so important an ollice ol transmission, it is apparent that this 

 motor area must be in direct communication with adjacent and more distant 

 portions ol the cerebral hemispheres. Siich communications ma\ be dis- 

 cerned 111 the many association libers, both ol the long and short Aariety, 

 \\ hich link the \ arious conxolutioiis together. The important Icature conei'rn- 

 ing the motor area itsell is its capacity to sublimate into ellecti\'e lormulas 

 and motor patterns all ol that great stream ol atlereiit impulses which enters 

 the brain. These motor patterns determining the externalized ex]:)ression ol 

 the animal's behaxior lind their ultimate con\e\ance b\' way ol the pyram- 

 idal system. It this system should actuall\ keep pact' with the expansions 

 of the cerebral lu-misphere by manifesting a corresponding increment in 

 size, it would soon become too extensixf lor either the bram stem or the 

 spinal cord. That it does not vary diri-ctl\ with the rate ol exi:)ansion in the 

 cerebral cortex is evident, although cortical exj^ansion docs exert a delinite 

 inllueiice in increasing the ]nramidal \<)lume. It is possibk' lor the cerebral 

 cortex to increase tenlold while the pxramidal system which conxeys the 

 imj)ulses tor externalizing the actixities ol the entire neopallium, adds but a 

 small traction ol increment to its size. Thus, the ]:)yramidal tract actuallx' 

 rellects the ellort toward condensation which is everywhere imposed upon 



