MIDBRAIN AND THALAMUS OF NECTURUS 289 



directly with the lateral geniculate body and pulvinar of mam- 

 mals, which are chiefly cortical dependencies. The medial 

 geniculate body is represented in a quite undifferentiated form 

 in the cuadal end of the pars dorsalis. 



The pars ventralis thalami 



This region is dominated by the great somatic efferent 

 systems leading downward into the cerebral peduncle (tractus 

 thalamo-peduncularis ventralis, page 263). It receives nervous 

 impulses (1) from the tectum mesencephali by fibers of the 

 tractus tecto-thalamicus et hypothalamicus cruciatus both before 

 and after their decussation in the postoptic conmiissure (p. 257) ; 

 (2) from the pars dorsalis thalami directly (tractus dorso-vent- 

 ralis thalami, page 266) and also by way of the tractus thalamo- 

 hypothalamicus et peduncularis cruciatus from the same side 

 and from the opposite side (p. 259) ; (3) from the lateral walls 

 of the cerebral hemispheres by way of the lateral forebrain 

 bundle (tractus strio-thalamicus, page 267). 



The neurons of this region have long, thick, widely branched 

 dendrites which are contorted but not varicose or thorny, the 

 axon arising from the base of the dendrite or one of its chief 

 branches (figs. 27, 28, 42 to 45). These dendrites spread out 

 among the fibers of the tracts mentioned in the preceding para- 

 graph and into the surrounding regions, notably into the pars 

 dorsalis thalami and the nucleus of the tuberculum posterius. 

 The tecto-thalamic and lateral forebrain connections appear to 

 be the most important sources of nervous impulses for this part 

 of the brain. Figure 27 illustrates the way in which dendrites 

 of these neurons turn ventrally to accompany the fibers of the 

 tractus tecto-thalamicus et hypothalamicus cruciatus as they 

 approach their decussation. They are siinilarly related to the 

 fibers of the tractus thalamo-hypothalamicus et peduncularis 

 cruciatus. Some of these dendrites appear to cross the medial 

 plane in the commissure. 



In general, the pars ventralis thalami of Nectui'us appears 

 to be a center devoted to the collection of nervous impulses 



