848 MAN 



formation contimu's upward and forms a base also for the superior colliculus. 

 while between ttu' mferior and superior eollieuli a dorsal extension oi the 

 retieular formation passes around the dorsal surfaee of the brain stem to und 

 by meeting its fellow of the opposite side at the midline, thus eompletely sur- 

 rounding the eentral gray matter of the meseneephalon. At the cephalo- 

 lateral extremity of the inferior eollieulus the retieidar formation is tunnelled 

 obliquely ventrad, cephalad and somewhat laterad by the course of the 

 inferior brachium which connects the inferior colliculus with the mesial 

 geniculate bodx. This channel lies partl\ on the surface but at times it is 

 bridged by extensions of the reticular formation which pass \entrall\ iiom 

 the portion lying immediately beneath the superior colliculus, and dorsally 

 from the part of the reticular formation forming the ventral lip of the 

 groove. 



AboN'e the point at which the superior cerebellar peduncle decussates, 

 the retieular formation adjacent to the raphe gradually increases in amount 

 ventrally and laterallx until it completely surrounds the decussated superior 

 cerebellar ijeduncle w hich is now terminating in the red nucleus and forming 

 a part of its capsule. This portion of the reticular formation surrounding the 

 red nucleus and its capsule is continued u])ward through the meseneephalon 

 and becomes continuous with the zona incerta ot the diencephalon. In the 

 upper j:)ontile and lower mesencephalic region the reticular tormation lormmg 

 the ventral lip of the channel containing the inferior brachium is continued 

 upward as a lateral support for the substantia nigra, in the mesencephalon, 

 as the lateral portion of the reticular formation is passing upward to luse with 

 the zona incerta of the (lience])halon, it sends out a lateral extension which 

 passes dorsal to the substantia nigra and ventral to the mesial geniculate 

 body, thus separating these two structures. This extension of the reticular 

 formation is cur\ed \entrall\ into the angle between the mesial geniculate 

 body and the mesial portion of the diencephalon to a narrow, tapering ])oint. 



