294 C. JUDSON HERRICK 



following order, from within outward. At the boundary be- 

 tween the gray and white layers are the brachium conjunctivum, 

 tractus tecto-peduncularis profundus, and tractus tegmento- 

 interpeduncularis. These are largely unmyelinated. Next to 

 these are the dorsal tegmental fascicles, and farther ventrally 

 the ventral tegmental fascicles, the fasciculus longitudinalis 

 medialis, the tractus thalamo-peduncularis cruciatus from the 

 postoptic commissure, and the tractus strio-peduncularis from 

 the lateral forebrain bundle. External to these are the tecto- 

 bulbar tracts and the superficial tecto-peduncular fibers. Be- 

 low the isthmus region at the level of the auricular lobes of the 

 oblongata the acoustico-lateral lemniscus and other lemniscus 

 systems come to lie superficially of all these tegmental tracts. 



Correlated with the fact that most of the nervous impulses 

 which descend through the midbrain of Necturus are interrupted 

 by at least one synapse under the tectum, we find three specially 

 differentiated regions, one above and one below the fovea 

 isthmi and one (the interpeduncular nucleus) extending 

 throughout the length of this region close to the mid-ventral 

 plane. These will next be reviewed. 



The nucleus of the tuberculum posterius. This ventricular 

 eminence lying dorsally and rostrally of the tuberculum post- 

 erius (figs. 63, 64, 68, nuc.tuh.p.) is an important coordination 

 station for assembling various types of descending motor fibers. 

 Its neurons are large, with long widely branched dendrites 

 (figs, 7, 8, 9, 23, 24, 30, 59, nuc.tuh.p.). In the dorsal part 

 of this eminence and extending somewhat farther dorsally than 

 its border, are the very large neurons of the nucleus of the pos- 

 terior commissure and fasciculus longitudinalis medialis (see pp. 

 252, 271 and figs. 8, 23, 59, 60, nuc.com. post.), which seem to be 

 more specialized members of the same functional complex, 

 though the ventricular eminence itself is not produced by these 

 neurons. There is a very slight ventricular eminence running 

 transversely just behind the nucleus of the tuberculum posterius 

 which is formed by these large cells and by the commissure itself 

 (figs. 63, 64 and compare figs. 7 and 8). 



