RECONSTRUCTION OF HUMAN 840 



The niori' nu'sial reticular niass continues upward in rather an irrei^ular form 

 and fuses with the retieuhir lorniatioii ol the diencephalon, gra(hiall\ giving 

 place to the mesial group ol thalamic nuclei. 



The Arciform and Pontile Nuclei 



The arciform nucleus consists of a thin lamina of gray matter lying 

 directU ventral to the p\ ramidal tract and beneath the surface of the brain 

 stem. It consists of an irregular gra\ lamina which in \an')iis parts ol its 

 course sends prolongations laterally around the \entrolateral surlace ol the 

 stem. It increases rather rapidly in width until it reaches the mid-oli\arv 

 region where it achieves its greatest lateral expansion, the lateral limit of this 

 extension being the jxisloliiury sulcus. From this point the arcitorm nucleus 

 rapidU diminishes in size and comes to a termination slightlx' caudal to the 

 origin of the pontile nucleus. The arcilorm nucleus extends mesially around 

 the |)\ ramidal tract at some points into the ventromesial sulcus ot the brain 

 stem, but in the greater jjortion of its extent it stops short of the ventro- 

 median sulcus. Its dorsal surface corres])oncls accurately with the conlorma- 

 tioii of the p\ ramidal tract which immediately underlies it, and at its lateral 

 extremity it a|)proaches the lundLis oi the inferior oli\arv nucleus and also 

 further dorsally comes into direct relationshi]) with the reticular lormation. 



The pontik' nucleus first appears at about the middle of tlu' mam olivary 

 nuch'us at a le\el slightl\' abo\e the termination oi the arcilorm nucleus, as 

 a small colk'ction of gra\' matter situated mesial to tlu' |)\raniidal trad. It 

 contiiUK's upward in this position, steadily extending laterally both \-entral 

 and dorsal to the pyramidal tract to which it is ch)scly appHcci, until the 

 uppt'r limit ol the mierior oli\ary nucleus is reached. I-rom this point the 

 |3ontile nucleus incrt'ases \ery ra])idl\ m size and comple\it\, becoming 

 complete mesiall\ and surrounding the |)\ ramidal tract \entrall\, mesially 

 and dorsalU with an unbroken sheath ol nuclear material. The pyramidal 



