244 C. JUDSON HERRICK 



marginal optic tract passed dorsalward across the rostral end 

 of the pars dorsalis thalami, it traverses this area superficially 

 and sends numerous collaterals into it (fig. 54, p.o.th.). There 

 are no complete neurons in this area, which is simply an entangle- 

 ment of the richly arborized dendrites of neurons whose cell 

 bodies lie in the underlying gi'ay layer, together with terminal 

 arborizations from collaterals of the fibers of the optic tract 

 and other types of fibers (figs. 25, 26, 40, 42 to 45, 48, 54, 56, 

 57, 65, p.o.th.). The character of these other types of fibers is 

 not altogether clear. They lie at deeper levels than those of 

 the marginal optic tract and exhibit free endings of a different 

 morphological type from the collaterals from the latter tract 

 (figs. 26, 56, 57, p.o.th.). They may be derived either from the 

 axial optic tract or from the tractus tecto-thalamicus et hypo- 

 thalamicus cruciatus by way of the postoptic commissure (p. 

 256). This area of neuropil contains synaptic connections 

 between the fibers of the optic tract and neurons of the thala- 

 mus, and is, therefore, functionally comparable with the optic 

 centers of the mammalian thalamus in spite of its anomalous 

 position near the rostral end of the thalamus. Since this area 

 seems to constitute the only optic receptive center in the 

 thalamus, I shall term it simply the optic part of the thalamus 

 {pars optica thalami). 



This neuropil of tlje pars optica thalami is reached by den- 

 drites from rather widely separated parts of the stratum griseum 

 of the thalamus, including the pars dorsalis farther caudad and 

 the pars ventralis ; but there is one group of cell bodies producing 

 a slight ventricular eminence between the eminentia thalami, 

 the pars ventralis thalami, and the pars dorsalis thalami whose 

 dendrites seem to be especially related to the ventral part of 

 this area, which has already been referred to (p. 230) as the 

 nucleus of the pars optica thalami (figs. 42 to 45, 48, 63, 64, 

 65, 67, 68, nuc.p.o.th.). 



At the level of the posterior commissure the marginal optic 

 tract enters the midbrain roof, passes inward toward the mid- 

 dorsal line, and distributes by widely spread free arborizations 

 of its fibers throughout the dorso-medial part of the tectum 



