MIDBRAIN AND THALAMUS OF NECTURUS 261 



lateral lemniscus (Im.) and those of the tractus tecto-thalamicus 

 et hypothalamicus cruciatus. They sweep forward across the 

 lateral aspect of the thalamus in diffuse formation dorsally and 

 rostrally of the two tracts last mentioned and appear to end 

 without decussation in both the pars dorsalis and the pars 

 ventralis thalami, chiefly in the former (figs. 5 to 8, 20, 28, 37 

 to 40, 55, 56, 66, tr.t.th.r.). 



15. Tractus tecto-habenularis 



Horizontal sections show a compact strand of unmyelinated 

 fibers passing close to the mid-dorsal line between the rostral 

 end of the tectum mesencephali and the habenula. The cell 

 bodies from which these fibers arise are not impregnated in our 

 preparations. Weigert sections show that there are some mye- 

 linated fibers among them. 



Associated with these fibers is a small number of myelinated 

 and unmyelinated fibers which pass in diffuse formation or in 

 dense fascicles through the commissura tecti diencephali and 

 are termed tractus tecto-habenularis cruciatus. The unmyelina- 

 ted component of this tract is seen at its decussation in the 

 commissura tecti in figure 28 (tr.t.hab.c), and this bundle can 

 be followed in the series of sections forward into the habenula 

 (fig. 25, tr.t.hab.c). In horizontal Golgi sections some fibers of 

 this system are seen to arborize freely in the pars intercalaris 

 diencephali and here to send collateral branches into the com- 

 missura tecti diencephali (fig. 37, tr.t.hab., on the right side). 

 This arrangement suggests that the fibers pictured arise in 

 the habenula and terminate in the pars intercalaris and tectum; 

 but the same series of sections shows farther ventrally fibers of 

 this tract extending from the tectum forward through the whole 

 length of the pars intercalaris and ending by richly branched 

 free arborizations in the habenula (fig, 38, tr.t.hab.), suggesting 

 their origin from the tectum. 



