MIDBRAIN AND THALAMUS OF NECTURUS 287 



rupted in the tectum mesencephali and neurons of a higher order 

 carry on the nervous impulses to the thalamus. This latter 

 arrangement appears to be the primitive one, and as we ascend 

 the phylogenetic series an increasingly larger proportion of the 

 lemniscus fibers effect direct connection with the thalamus. 



The pars dorsalis thalami exhibits little evidence of separate 

 localization of specific nuclei for the optic, acoustic, somesthetic, 

 and other functional systems which enter it, though there are 

 some indications of the early stages of such functional localiza- 

 tion. Its posterior portion receives the scanty thalamic ter- 

 minals of the spinal and acoustico-lateral lemnisci; its middle 

 and most massive portion receives the greater part of the strong 

 uncrossed tecto-thalamic tract and possibly hypothalamic fibers 

 representing the tractus mamillo-thalamicus (p. 265) ; its 

 anterior portion contains the sharply circumscribed neuropil 

 to which we apply the name pars optica thalami. This neuropil 

 receives collaterals from fibers of the marginal optic tract, and 

 probably also of the axial optic tract, as has been more fully 

 described on page 244. It may also receive some terminals from 

 the anterior part of the tractus tecto-thalamicus et hypothal- 

 amicus cruciatus after decussation in the postoptic commissure 

 (p. 256). Dendrites of neurons from all adjacent parts of the 

 thalamus may enter this neuropil, but there is one group of 

 neurons whose dendrites seem to be related exclusively with 

 this area. These neurons form a slight ventricular eminence 

 in the rostral end of the pars dorsalis thalami immediately 

 caudad of the eminent ia thalami, and are termed the nucleus 

 of the pars optica thalami (figs. 42 to 45, 63, 64, 65, 67, 68, nuc.p.o. 

 ih.). The axons from this nucleus are directed caudad through 

 the pars ventralis thalami, thus forming the most ventral fibers 

 of the tractus thalamo-peduncularis dorsalis (figs. 42 to 45, 48). 



The neurons of the more massive middle portion of the pars 

 dorsahs thalami have longer dendrites than those of the nucleus 

 of the pars optica. Some of these dendrites reach the pars 

 optica, as just mentioned; but most of them spread out among 

 the terminals of the tractus tecto-thalamicus rectus and other 

 fibers of the overlying stratum album. Their axons in part 



