844 MAN 



appears slightly ahoNc the point ol ojicning ol the iourth \entnele, inereascs 

 rapidly in size both \-entrocl<)rsall\' and mesiolaterallx , and eontnuies upward 

 as a prominent mass of gray matter. Above the point at whieh the pontile 

 luielei are first seen the aeeessory portions of tlu- luieleus disappear and the 

 luieleus itself eontracts, produeing a dome-shaped strneture which lies within 

 the more suj)er(ieially placed masses of gray matter representing the caudal 

 portion of the pontile nuclei. Laterally the mass of the inferior oli\ary nucleus 

 is separated from the ventral surface of the reticular formation by a con- 

 siderable space which is occupied by the continuation upward of the lateral 

 white column of the spinal cord. 



The Reticular Formation 



This mass of nuclear material representing the matrix from which many 

 specialized nuclei have differentiated does not bulk so largely nor attain the 

 apparent importance which it manifests in the lower members of the series. 

 It appears as a more or less quadrilateral mass of gray matter extending 

 obliquely from the xentrolateral angle of the oblongata, dorsomesially to its 

 point of conllueiice w ith the body of the gra\- matter and at a higher level 

 the floor of the Iourth \entricle. Its \entromesial surface is more or less 

 irregular, presenting a concavity at the point where the inferior olivary 

 nucleus comes into close relationship with the reticular formation. In the 

 lower portion of llu' oblongata it is separated from its fellow of the opposite 

 side by a consick-iable distance which is occupied b\ the longitudinallx 

 coursing grou|)s of medlanly disposed libers. As these structures gradually 

 seek other [positions m t!u' brain stem tlu' reticular lormalion apj^roaches the 

 midline and its dorsonusial extremity becomes w ider and Hatter as it comes 

 into contact with tlu' raphe. In tlu' lowest portion ol the reconstruction the 

 ventrolateral i'xtremit\' of the reticular lormation is continued \entrally and 

 laterally into two prolongations, the ventral one of which is continuous with 



