6i4 THE HIGHER ANTHROPOIDS 



The Reticular Formation 



The rt'tu'iilar lorination aj^j^cars at a It've-I corrcspondinp; to the hcpiiining 

 of the pyramidal decussation as an aeeuiiiulatiun ol white and <2;i"a\' matter 

 lateral to the remnants of the ventral gray column. It lies in the meshes 

 iaetween the decussating pyramidal fiix-rs. As the pyramidal tracts gradually 

 fuse in tlu' median line, this formation assumes greater prominence. The 

 ventral gray columns, as they ascend, are increasingly broken up by the 

 bundles ol the pyramidal tract and therefore acquire an arrangement charac- 

 teristic ol the ri'ticular formation, withm w Inch latter they linally disaj)pear. 

 The reticular lormation becomes more massi\e in the metencephalon in w Inch 

 segments are lound those \arious condensations lorming important neuraxial 

 nuclear masses. In outline the reticular lormation is more or less rectangular, 

 presenting lateral, mesial, ventral and dorsal i)ordcrs. The lateral border 

 is covered oiil\ b\ white libers and extends almost to the periphery of the 

 cord, l\ ing l)etween the inferior olixarx nucleus and the substantia gelatinosa 

 trigcmini. its \cntral aspect is extensive and receives the dorsal surface of 

 the inferior olivary complex. Its mesial border is located near the raphe 

 between the dorsal extremities of the mlcrior oli\ary nucleus and the ventral 

 gray matter, while its dorsal extent is apj^lied to the ventral surface ol the 

 central gray matter as this structure spreads out to form the lloor ol the lourth 

 ventricle. This disposition is continued upward, the reticular lormation 

 forming tlu' mam mass ol the tegmentum ol the brain stt'in. It contains 

 various fiber tracts and forms the matrix lor tlu' development ol sexeral 

 nuclear collections. It provides the general support for the entire tegmental 

 structure. The two mesial surfaces are |)laced opposite one another, each 

 being separated from its fellow b\ the longitudinal bundles which make u]j 

 the raphe. 



With the de\'elo|:)mciit of the pontiK' nuck'i the ventral surface of the 

 reticular formation conu's into relationship with these structures. 



