THE BRAIN OF MODERN MAN 819 



a portion of the sccondarx cochlrar [jatliway. Situated in the lateral extrem- 

 ity of the trapezoid l)ocl\ is the upper pole of the superior olivary body, 

 (SO), a relay station in the pathway of auditory conduetion Ironi whieh 

 some hht'rs arise to take a eourse inward and haekward m the chrec- 

 tion of the nueleus ahdueentis. These libers form the pedunc'le ol the su|KM-Ior 

 olivary body. They serve the ])urpose of transmittinii; auditory impulses 

 from the superior oIi\e to tlu' nueleus of" the sixth ner\e, in order that the 

 eyes may be directed toward the souree of any sudden sound oeeurring u])on 

 one side or the other of the indixidual. This pedunek' ol the supt-rior olix'ary 

 bodv develops in all primatt's, haxinii in man a representation as proiiouneed 

 as any of the lower forms. Immediate rellex cooperation Ix'tweeii ear and 

 eyes is as much a human nvcd for ]M-ompt dettxtion ol a|j[:)roaching danger as 

 it is essential to the protection of the upva. 



The body of the central <j:ray matter, a thin layer lormint; the lloor 

 of the fourth ventricle, is pr()lon<;-ed laterally as a tenuous stratum beneath 

 the subependymal substance over the walls of the \entricular space which 

 now are formed by the massive bundles of the superior and middle certbellar 

 peduncles. The ventricle itself is somewhat more restricted, due to the iact 

 that it is ap]iroachin<,^ the caudal orifice ol the Sylvian aqueduct. In the 

 \entromesial |)ortion of the ct-ntral <i;ray matter is the specialized aggregation 

 ol' ner\e cells forming the nucleus abducentis (Nab). This is the nucleus 

 of the sixth ner\e which supplu-s the external rectus muscle ol tlu' eye, the 

 fundamental center of all ocular moxt'ments in lateral gaze. The emergent 

 fibers of the sixth nerxe make their wa\ forward from the nucleus toward the 

 pons \ arolii. It is sonuw hat unusLial to observe as in this section three por- 

 tions of the emergent libers in the intramedullary course of the sexenth nerxe 

 whieh supplies the i'acial musculaturi'. The microtome was fortunate in 

 rexealing the second portion passing along the mesial aspect ol the nucleus 

 abdiici'iitis, the third ixution, the genu facialis, and the lourth portion as 



