256 C. JUDSON HEREICK 



they are said to connect with the ventral regions of the mid- 

 brain roof. Since the optic tracts are entirely absent in Pro- 

 teus and are greatly reduced and unmyelinated in Necturus, 

 it seems improbable that these coarse heavily myelinated fibers 

 are related to the optic apparatus. They probably are in no 

 way functionally related with the associated unmyelinated 

 fibers to be described immediately. In this connection it 

 should be remembered that the area here termed colliculus su- 

 perior has numerous other functional connections in addition to 

 the optic tracts, e.g., it receives fibers from the spinal and acous- 

 tico-lateral lemnisci and from the mesencephalic V root. 



Golgi sections show that there is a large number of unmyelina- 

 ted fibers associated with the coarse myelinated fibers last 

 described and, before their decussation, following the same 

 course, that is, in close association with the marginal optic 

 tract (figs. 17, 37, 54, 55, 57, 67, tr.t.th.h.c.a.). After their 

 decussation these fibers are associated with fibers derived from 

 the hypothalamus and with the basal optic tract (see p. 242) 

 and they cannot easily be followed. In one sagittal series by 

 the Golgi method, in which the marginal optic tract is entirely 

 unimpregnated, this tract as it descends from the tectum is 

 well impregnated (fig. 56, tr.t.th.h.c.a.), and slightly detached 

 from this fascicle are scattered fibers apparently belonging to 

 the same system which end by free arborizations in the pars 

 optica thalami (fig. 56, tr.th.h.c.a.x.). These may be interpreted 

 as fibers of this tract arising in the tectum and ending in the 

 thalamus after decussation in the postoptic commissure, though 

 they may have other significance (see p. 244). 



2. Pars posterior. This tract comprises the most compact and 

 most easily followed component of the postoptic commissure 

 complex. Its fibers are related to that portion of the tectum 

 mesencephali which receives lemniscus fibers from the spinal 

 cord and oblongata. Its myelinated fibers are of small size 

 but have dense myelin sheaths, so that they stain intensely in 

 Weigert preparations. This rather compact tract in the more 

 dorsal part of its course is mingled with fibers of the acous- 

 tico-lateral lemniscus as these spread out to their termini in 



