RECONSTRUCTION OI HUMAN 847 



niesenct-phalic tcgmciUiini. This (.hanin'l loinu'd h\- the suptTior cerebellar 

 ])e(lunele is lar<ie, well delmi'd aiul (lislinet. It is liniilecl iiiesiall\ b\ that 

 portion ol tlu' reticular loi'iiiation which is 111 contact with the lloor ol the 

 iourth x'cntncle, and tlu' ac|ucdiK't ol Syl\ lus, while laterally it is hrst uncov- 

 ered by the reticular formation but as it sinks (kH'|)er and di'cpcr into the 

 tegmentum it is gradually surrounded b\- the thm lateral txtt-nsions ol the 

 reticular lormation alread\ mentioned. As the superior cerebellar peduncle 

 penetrates the tegnu'iitum and mo\ es mesiall\ and \i'ntrall\ it progressively 

 reduces the amount ol reticular lormation betwc'ii it, the lloor ol the Iourth 

 ventricle and tlu' nu'dian rai)lu-, until m the upper portion ol the mctenceph- 

 alon the peduncle reaches ihv midline and completelx obliterates that part of 

 the reticular lormation which lies mesial to it. In this region the reticular 

 lormation is divided b\ the superior cerebellar peduncle into two jjortions, a 

 thm lamina \i,'ntrall\ \\ Inch has already been mentioned as bi'ing m contact 

 with the deep layer ol the pontiU' nucU'us and a dorsomesial triangular mass 

 conca\'e latcrall\ which rests mesiallx on the raphe and dorsall\' on the 

 \entricLilar gray matter. The ventrolatt'ral lamina ol the reticular lormation 

 luses dorsall\ with the dorsomesial triangular mass at the dorsolateral angle 

 of the brain stem at the Icn'cI of the ujjpcr port ion of the fourth \ entricK' and 

 the beginning ol the acjueduct of Sylvius. As the superior cerebellar peduncle 

 moves ventromesiall\- toward its decussation this area ol lusion between the 

 ventrolateral lamina and the dorsomesial mass of the reticular formation 

 becomes more massi\e. 



In this vicinity a second tunnelling of this portion of the reticular lorma- 

 tion takes place by the course of tiie lateral fillet from thi- \ tntrolatcral angle 

 ol the brain stem as this structure approaches the inlerior colliculus. The 

 reticular lormation lying immediatelx subiaceiit to the inferior colliculus 

 assumes considerable proportions and forms a foundation or support for this 

 colliculus at its \t-ntrolatt'ral c\tremit\. This lateral mass of the reticular 



