MIDBRAIN AND THALAMUS OF NECTURUS 295 



Some of the dendrites of the neurons of the nucleus of the 

 tuberculum posterius are very long, reaching dorsalward in 

 company with the axons of the tractus thalamo-peduncularis 

 to the pars dorsalis thalami and even to the pars intercalaris; 

 others reach forward and ventralward as far as the nuclus of 

 the tractus pallii immediately behind the postoptic commmissure. 

 These dendrites receive nervous impulses by way of extensive 

 tracts from the hypothalamus, from the medial and lateral 

 forebrain bundles, from the thalamo-pedunclular tracts, from the 

 posterior commissure, and from the midbrain and thalamus by 

 way of the postoptic commissure. The efferent discharges from 

 this region go in part to the nuclei of the III and IV nerves and 

 in larger part into the dorsal and ventral tegmental fascicles and 

 the fasciculus longitudinalis medialis. 



Laterally of the tuberculum posterius is an elongated super- 

 ficial area of dense neuropil, the area lateralis tegmenti (figs. 

 8, 9, 10, 31, 32, 54, 55, a.l.t.), which is reached by dendrites with 

 thick bushy terminals from neurons of the nucleus of the tuber- 

 culum posterius. In this neuropil are the much branched 

 termini of the thick unmyelinated axons of the supposed basal 

 optic bundle (p. 242). It is also reached by fibers of the medial 

 forebrain bundle, by fine freely branched axons from the motor 

 tegmentum farther caudad (especially well shown in some of 

 our preparations of larval Amblystoma), by axons from the 

 hypothalamus and from the pars intercalaris diencephali. 

 There are indications in vom Rath and Cajal preparations that 

 the fibers ascending into it from the motor tegmentum are 

 derived from the ascending visceral tz'act (visceral lemniscus) 

 and superior secondary visceral nucleus, but of this there is no 

 clear demonstration. If these latter indications should be con- 

 firmed, it would identify this neuropil as in part a visceral (olfac- 

 to-gustatory) correlation center of a higher order. In the frog 

 I have seen in the corresponding region quite superficially and 

 immediately rostraUy of the superficial origin of the III nerve 

 a small nucleus, whose cells are arranged as a hollow sphere 

 surrounding a core of dense neuropil, which may be a more 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 28, NO. 2 



