THE SYSTEMS OF FIBERS 281 



related with the central nucleus of the isthmic tegmentum. These are 

 doubtless precursors of the hypothalamic component of the mam- 

 malian f. longitudinalis dorsalis of Schiitz. 



Tegmental fascicles of groups (5) to (10) contain fibers of diverse 

 origin, and there is much anastomosis of their finer fibers. The 

 coarser fibers, however, are well fasciculated in an arrangement 

 which seems to be determined primarily by the terminal distribution 

 of the descending systems. Their analysis has been clarified by the 

 previously published embryological studies; see particularly the gen- 

 eral survey ('396) and for the decussating systems the description of 

 the postoptic commissure of the adult ('42). The data upon which 

 this summary of the composition of the several fascicles is based are 

 to be found mainly in the two papers just cited. 



Ventral fascicles (5). — This is a group of large fascicles mainly com- 

 posed of fibers (chiefly myelinated) descending from the ventral 

 thalamus and peduncle. Most of them are uncrossed, but some 

 decussate in company with those of tr. tecto-bulbaris cruciatus in the 

 ventromedial fascicles of group (1) ('396, p. 546 and fig. ^S,f.v.t.{5)). 

 They arise from all parts of the ventral thalamus and peduncle, and 

 most of them terminate in the alba of the isthmic and trigeminal 

 tegmentum, where they are in synaptic contact with the large cells 

 of this region. The thicker and more heavily myelinated fibers are 

 closely fasciculated laterally of the ventral angle of the ventricle, and 

 many of these descend for undetermined distances in the f . longitu- 

 dinalis medialis. The origin of these fibers from the ventral thalamus 

 is shown in figures 31 and 32. At those levels similar thick fibers enter 

 fascicles (5) from large cells of the peduncle, but they are not drawn 

 in these figures. Figures 6 and 18 show fibers entering the f. longi- 

 tudinalis medialis from large cells of the dorsal and ventral parts of 

 the peduncle. The former (shown but not named in fig. 32) are in 

 synaptic connection with terminals of the posterior commissure and 

 doubtless correspond with the mammalian nucleus of Darksche- 

 witsch (p. 217). The more ventral large cells of the peduncle (fig. 

 31) correspond with the interstitial nucleus of Cajal. In Amblystoma, 

 unlike Necturus and most other vertebrates, the f. longitudinalis 

 medialis is definitely organized only spinalward from the isthmus 

 ('36, p. 334). 



The fascicles of group (5) also receive some fibers from other sys- 

 tems, as will appear below. For their courses as seen in horizontal sec- 

 tions see figures 27-32; in transverse sections, figures 91-94 and '36, 



