THE COMMISSURES 303 



pothalamicus posterior. The remainder of the commissure of the 

 tubercuhim and the anterior part of the ventral tegmental commis- 

 sure comprise the ansulate commissure of the literature. 



In Amblystoma I have recognized four chief components of the 

 commissure of the tubercuhim posterius ('36, p. 305 and fig. 2), ar- 

 ranged as shown in sagittal section in figures 81, 104; in horizontal 

 sections in figures 28, 29, 30, 31 (component 4 being here marked, 

 tr.t.b.c.l.); and in transverse section in figure 94 (components 2 and 

 3 shown here, components 1 and 4 being shown in a neighboring 

 section, '36, fig. 11). The connections of these components are as 

 follows : 



1. This is the partial decussation of unmyelinated fibers from the 

 dorsal (mamillary) part of the hypothalamus to the interpeduncular 

 nucleus ('36, p. 338 and figs. 3, 8, 11-13). Most of them enter ventral 

 tegmental fascicles of group (2) as marked tr.mam.inp., on figures 19, 

 27-30, 71, 81, 92, 103. 



2. This component contains partly myelinated fibers from the 

 dorsal hypothalamus to the peduncle and tegmentum, accompanied 

 by many uncrossed fibers and by fibers conducting in the reverse 

 direction from the peduncle to the hypothalamus (p. 278; figs. 8, 21, 

 tr.mam.feg., 23, tr.mam.ped.). Most of them spread in the alba of the 

 peduncle or enter ventral tegmental fascicles of group (3). Another 

 system is tr. hypothalamo-peduncularis et tegmentalis from, the ven- 

 tral part of the hypothalamus to the peduncle and tegmentum (fig. 

 21, ir.hy.teq. and fig. 23, tr.hy.ped.). There is interchange of all these 

 fibers with those of component 1 and with other tegmental fascicles. 

 They are marked tr.mam.ped., tr.mam.teg., and (3) on figures 18, 21, 

 23, 27-31, 79, 82, 92, 94, 103. 



3. The third component contains heavily myelinated fibers from 

 the tectum, pretectal nucleus, and dorsal thalamus to the peduncle, 

 accompanied by many uncrossed fibers — tr. tecto-peduncularis cru- 

 ciatus {tr.t.p.c.) and tr. thalamo-peduncularis cruciatus (tr.th.p.c), as 

 elsewhere described (p. 223; '42, p. 267). These are shown on figures 

 12, 15, 18, 22, 29-36, 94, 103. 



4. The dorsal component of this commissure is larger than the 

 others and is composed chiefly of tecto-bulbar and tecto-spinal fibers 

 from the anterior part of the tectum opticum, marked tr.t.b.c.l. on 

 the figures. Its fibers after oblique decussation enter ventral teg- 

 mental fascicles of group (1) (see p. 277 and figs. 12, 27-36, 93, 94). 



