MIDBKAIN AND THALAMUS OF NECTURUS 257 



the tectum. In following its course from the tectum it is 

 seen to run forward in the intermediate level of the stratum 

 album ventrally of the neuropil related to the optic tract, or 

 optic tectum (figs. 7 to 10, tr.t.th.h.c.p.), and then similarly 

 across the caudo-lateral face of the thalamus (figs. 3 to 6) min- 

 gled with, fibers of the tractus tecto-thalamicus rectus (fig. 56). 

 Finally it turns abruptly ventralward to decussate near the 

 rostral border of the chiasma ridge (fig, 67). It runs parallel 

 with the optic tract, but farther caudad and at a somewhat 

 deeper level. 



Mingled with the myelinated fibers just described are many 

 without myelin sheaths (figs. 18, 19, 20, 27 to 30, 36, 37, 38, 41, 

 42, 43, 46, 52 to 58, 62, 67-, tr.t.th.h.c.p.). Golgi sections show 

 that many of these arise as axons of neurons of the tectum. 

 There are, however, some free endings in the tectum which 

 belong to this system (fig. 37). I conclude that the tract is 

 chiefly efferent with reference to the tectum; the free endings 

 may indicate that mingled with the efferent fibers are others 

 which are truly commissural between the two tecta 



From the nucleus posterior tecti strong fascicles of both 

 myelinated and unmyelinated fibers enter this system (fig. 37). 

 In Golgi preparations of Necturus.and larval Amblystoma there 

 is in this region a densely massed group of neurons whose den- 

 drites reach the lateral surface and there turn forw^ard among 

 the fibers of this tract. Slender axons of the postoptic commis- 

 sure system enter this dendritic neuropil, but their exact re- 

 lations with these neurons have not been determined. 



As the fibers of this pars posterior cross the caudal end of the 

 pars ventralis thalami before their decussation, they give off 

 numerous fine contorted collaterals which form an intricate 

 neuropil within which dendrites from neurons of the pars ven- 

 tralis thalami ramify (fig. 27). Thick contorted dendrites of 

 these neurons also turn ventrad within this tract for long dis- 

 tances. These fibers after their decussation cannot be distin- 

 guished with perfect certainty from other components of the 

 postoptic commissure. Apparently they spread out both rostrad 

 and caudad of the decussation into the pars ventralis thalami, 



