THE SYSTEMS OF FIBERS 285 



tioned, this large and complicated system of crossed and uncrossed 

 fibers was overlooked until early larval stages revealed its essen- 

 tial features ('396, p. 546). These thick fibers arise in all parts of the 

 ventral thalamus. Some of them decussate in the postoptic com- 

 missure and enter fascicles (4), (6), and (8). Others descend uncro sed 

 in all these groups and also in group (5). Golgi sections show that 

 these tracts have numberless terminals and collaterals throughout 

 their lengths. The thicker fibers of group (8) course more dorsally 

 and terminate more laterally in the isthmic and trigeminal teg- 

 mentum. 



The composition of group (8) as it leaves the postoptic commissure 

 is shown in figures 25 and 26 and its further course in figures 27-33. 

 Figures 30-33 show uncrossed fibers from the ventral thalamus join- 

 ing the bundles of crossed fibers (compare '39&, fig. 2, and see the 

 concluding comment about fascicles of group (9)). 



Dorsal fascicles (.9). — These are composed chiefly of tr. strio-teg- 

 mentalis (fig. 101; '27, p. 287; '36, pp. 304, 335; '396, fig. 79), which 

 is the dorsal component of the lateral forebrain bundle, passing from 

 the dorsal nucleus of the primordial corpus striatum to the trigeminal 

 tegmentum. Some of these fibers decussate in the anterior commis- 

 sure, and a small number descend as far as the level of the VII nerve 

 roots, with terminals and collaterals along the way. 



These fascicles are easily followed in both horizontal and trans- 

 verse sections (figs. ^S-SS,f.lat.t.d. and (9)), and especially clearly in 

 sagittal sections (figs. 16, 19, 21, 72, 102; '36, fig. 5; '39, figs. 3, 7-12; 

 '396, figs. 1, 79). There is interchange of fibers between these fascicles 

 and those of groups (7a) and (8), and the thicker fibers of both 

 groups turn laterally in the trigeminal tegmentum close to the gray 

 and spray out dorsally of the V and VII nuclei, where they apparent- 

 ly activate both large and small tegmental cells. As the fascicles of 

 group (9) reach the posterior end of the ventral thalamus, they turn 

 sharply dorsad, parallel witlv the limiting sulcus, s, around the an- 

 terodorsal border of the nucleus of the tuberculum posterius. Here 

 those of (9) lie ventrolaterally of (8) (figs. 16, 91-94; '36, fig. 5), with 

 much anastomosis between them. The finer fibers of both groups 

 spread widely in the neuropil of the peduncle and dorsal and isthmic 

 tegmentum, and some descend in fascicles of group (7). The primary 

 function of groups (7), (8), and (9) appears to be the control of the 

 feeding musculature of the jaws and hyoid from the corpus striatum, 

 thalamus, and tectum. 



