226 THE BRAIN OF THE TIGER SALAMANDER 



CONCLUSION 



The internal structure of the optic tectum of Amblystoma and the 

 wide spread of its fibrous connections, both afferent and efferent, sug- 

 gest that in this animal the primary function of the tectal system is 

 visual control over movements of the body as a whole and, in par- 

 ticular, the orientation of the body and conjugate movements of the 

 eyeballs with reference to objects in the visual field. Such other local 

 visual reflexes as the animal possesses are probably organized else- 

 where. 



TECTO-OCULOMOTOR CONNECTIONS 



The visual location of food and enemies involves the innervation 

 of the muscles of the eyeballs, and our knowledge of the apparatus 

 employed in the conjugate movements of the eyes is scanty. The 

 anterior and posterior divisions of the crossed tecto-bulbar tract 

 decussate, respectively, in the vicinity of the nuclei of the III and IV 

 nerves (fig. 12). The anterior division {tr.t.h.c.l.) crosses in the ven- 

 tral medial tegmental fascicles which span the rather long distance 

 between the III and the IV nuclei. When these ventral fascicles — 

 group (1) of my analysis in 1936 — were first identified as tecto-bulbar 

 fibers, it was supposed that this anomalous arrangement was a provi- 

 sion for activation of the eye-muscle nuclei by collaterals from these 

 fascicles. No evidence of this has been found, but the myelinated 

 fibers of both tr. tecto-bulbaris cruciatus 1 and 2 are accompanied by 

 very many shorter, unmyelinated fibers, which pass from the tectum 

 to the III and IV nuclei of both sides. This tr. tecto-peduncularis is 

 shown diagrammatically in figures 18, 22, and 24 (and in more detail 

 in '42, figs. 14, tr.t.p.2., and 45, tr.t.p.c.2.). In addition to the tecto- 

 peduncular fibers in the alba, there is a deep series (tr. tecto-peduncu- 

 laris profundus, '42, p. 267 and fig. 14, ir.t.p.3.), which takes tortuous 

 courses in the neuropil of the grisea. From the anterior part of the 

 tectum and pretectal nucleus the well-defined tr. tecto-peduncu- 

 laris cruciatus and the uncrossed tr. thalamo-peduncularis dorsalis 

 superficialis may participate in the regulation of conjugate move- 

 ments of the eyes. 



If pupillary constriction is controlled from the pretectal nucleus, as 

 in mammals, there are two available pathways to the III nucleus, 

 first, by way of the pretectal components of the two tracts last men- 

 tioned and, second, by tr. pretecto-hypothalamicus (p. 296 and fig. 

 15, tr.pt.hy.) and thence to the III nucleus by tr. hypothalamo- 

 peduncularis (figs. 18, 23, tr.hy.ped.). 



