Special So\l\tic Afferent System 37 



The trapezoid body comes into relation with certain 

 masses of gray matter in the transverse part of its course, of 

 which the most conspicuous are the superior olivary nuclei. 

 These are more largely developed in the rat than in man — ^as 

 are also the cochlear nuclei — and have the form in this and 

 many other mammals of a lamina folded twice, so as to appear 

 S-shaped in cross section. Immediately medial to each is a 

 smaller lamina, the accessory superior olivary nucleus, and 

 some little distance medial to this again is a much less defi- 

 nitely circumscribed group of cells, the nucleus of the trapezoid 

 body. Some of the trapezoid fibres end in some one of these 

 nuclei, either before or after crossing the median plane, and 

 fibres arising from them pass into the lateral lemniscus. Many 

 of the fibres arising in the superior olivary nucleus, however, 

 run in a rather diffuse tract which may be seen passing dorso- 

 medially towards a small group of cells near the floor of the 

 ventricle, the nucleus of the sixth nerve^ (PI. X.). These 

 fibres mediate direct reflex movements of the eyes in response 

 to auditory stimuli. Other such fibres run through the reti- 

 cular substance from all the nuclei of this group to form short 

 reflex paths to other motor nuclei. 



The superior olivary complex of the rat, which has been analyzed as 

 follows by Fuse, contains more different elements than are enumerated 

 above. Besides the principal superior olivary nucleus, there are (1) the 

 accessory superior olivary nucleus, consisting of a small, dorso-ventrally 

 elongated gray mass and a largely-developed dorso-medial accessory 

 group of cells; (2) the nucleus of the trapezoid body; (3) the nucleus 

 praeolivaris internus; (4) the dorsal accessory nucleus of the principal 

 superior olive; and (5) the nucleus praeolivaris externus (poorly developed 

 in the rat). 



The accessory superior olivary nucleus receives trapezoid fibres 

 (mostly heterolateral) and gives off axons to the medial lemniscus and to 

 the homolateral nucleus of the trapezoid body. It also has connections 

 with its own medio-dorsal accessory cell-group, and with the nucleus 

 praeolivaris internus of the same side. The medio-dorsal cell-group is 

 larger than the rest of the nucleus, and is separated from the nucleus of 



^Fuse doubts whether they reach the abducens nucleus in the rat. 



