THE SYSTEMS OF FIBERS 277 



tral and that almost all fibers bordering the gray in the ventral 

 fascicles are myelinated. 



Ventromedian fascicles (1). — These are limited to the midbrain and 

 isthmus, extending spinalward from the commissure of the tuber- 

 culum posterius. Most of their fibers decussate obliquely in the ven- 

 tral commissure. They comprise 5 to 12 well-defined bundles of rather 

 thick myelinated fibers arranged close to the gray under the ventral 

 angle of the ventricle, with some admixture of unmyelinated fibers. 

 These fibers are derived from various sources, and they are variously 

 distributed. The bundles as definite anatomical entities are as- 

 sembled only in the space bounded approximately by the levels of the 

 nuclei of the III and IV cranial nerves. This distance is greater in 

 urodeles than in most other vertebrates, and the arrangement of 

 these fascicles found in Amblystoma has not been described in any 

 other species. In Necturus reduction of the optic system involves 

 corresponding shrinkage of these median fascicles ('36, p. 348), 

 though the general plan is similar. Three components of these 

 fascicles have been distinguished. 



a) Tractus tecto-bulbaris et spinahs cruciatus, pars anterior (fig. 

 12, tr.t.h.c.l.). — This is by far the largest component of the group. 

 Its fibers, chiefly myelinated, descend from the tectum and turn 

 spinalward in the mid-plane, here decussating in component 4 of the 

 commissure of the tuberculum posterius, and posteriorly of this very 

 obhquely in the ventral commissure ('36, pp. 303, 330, figs. 2, 7). 

 At the level of the nucleus of the IV nerve they turn laterally and 

 descend in tr. tecto-bulbaris et spinalis within the ventromedial alba 

 of the medulla oblongata. The course of this tract, as seen in hori- 

 zontal sections, is shown in figures 27-36 (for complete description 

 see '36, p. 340, and '42, p. 268). The posterior division of this tract 

 (figs. 27-36, tr.t.h.c.2.) does not enter the tegmental fascicles but 

 decussates transversely, at the level of the nucleus of the IV nerve. 



h) Tractus pedunculo-bulbaris ventralis cruciatus ('36, figs. 2, 7, 

 f.m.t.{l)hr, '39&, p. 582 and figs. 23, 24).— These fibers arise from the 

 ventral thalamus and peduncle and enter ventral fascicles (1). Here 

 they decussate obliquely, mingled with those of the crossed tecto- 

 bulbar tract; and after crossing they separate from the latter to 

 descend in the ventrolateral fasciculi of the medulla oblongata, and 

 some of them in the f. longitudinalis medialis, as described in the 

 references cited. 



