MIDBRAIN AND THALAMUS OF NECTURUS 269 



Tractus pallii. Still another component of the lateral fore- 

 brain bundle is a tract which I have recognized in the frog ('10, 

 p. 444 and fig. 41) and compared with Johnston's account of the 

 tractus pallii of fishes. This tract is well developed in all of 

 the urodeles which I have studied and its fibers are always 

 unmyelinated. It probably contains both ascending and de- 

 scending fibers, for I have seen numerous free arborizations at 

 both ends of the tract. It can be followed from the postero- 

 lateral wall of the cerebral hemisphere into the ventral part of 

 the cross section of the lateral forebrain tract (figs. 1 to 5, tr.pal.) 

 and then back to the level of the postoptic commissure. Near 

 the caudal end of this commissural complex the tractus pallii 

 fibers turn abruptly ventralward (fig. 5) and part of its fibers 

 decussate in the commissure. Both the crossed and the un- 

 crossed fibers are related to a fairly distinct nucleus in the hypo- 

 thalamus near the mid-plane and enclosing the most caudal 

 fibers of the postoptic commissure (fig. 6, nuc.tr. pal.). 



This tract may be the forerunner of the olfactory projection 

 tract of mammals. As described by Ram6n y Cajal ('11, p, 

 721), these fibers connect the pyriform lobe and amygdala of 

 the hemisphere with the motor tegmentum. This tract has 

 been seen by Johnston in the turtle ('15, figs. 16, 49, 52 to 54) 

 and by Miss Crosby in the alligator ('17, p. 373). The former 

 author ('15, p. 464, fig. 54) comments upon the fact that in 

 turtles, as in mammals, this tract reaches the tegmental region 

 of the midbrain, a connection which the tract in question in 

 Necturus does not make. 



Mingled with the axons of the lateral forebrain bundle are 

 long, slender dendrites; some of these come from the caudal 

 end of the ventro-lateral part of the cerebral hemisphere (striatal 

 area) and reach as far caudad as the chiasma ridge, others come 

 from the pars ventralis thalami and nucleus of the tuberculum 

 posterius and extend as far forward as the chiasma ridge. 



2) The medial forebrain bundle (fasciculus medialis telen- 

 cephaU). This is a broad connection between the medial wall 

 of the cerebral hemisphere and the hypothalamus. The fibers 

 are unmyelinated with a few myelinated fibers among them 



