222 THE BRAIN OF THE TIGER SALAMANDER 



probably smaller in Ambly stoma than in some other vertebrates, and 

 different opinions have been expressed about it. Eugen Frey's de- 

 scription ('38) of a large hypothalamic optic root in some amphibians 

 was evidently based on inadequate material and faulty observation, 

 as I have pointed out ('396, pp. 558-76; '41a, p. 498; '42, pp. 218, 

 233). The same is probably true of Geiringer's ('38) description of a 

 passage of fibers from the optic nerve directly into the hypophysial 

 tract. I have preparations that give this appearance, but more critical 

 examination disproves it. There is some evidence that in fishes and 

 amphibians efferent fibers go out from the preoptic nucleus through 

 the optic nerve to the retina ('336, p. 254); but I have not been able 

 to confirm this or the claim of some authors that efferent fibers go 

 out to the retina from the tectum. Both these connections may exist. 

 In the chiasma the coarser optic fibers are segregated from the 

 finer, and most of them take deeper courses in the optic tracts — the 

 axial bundles. As the optic tracts traverse the thalamus, they separate 

 into medial (dorsal) and lateral (ventral) tracts, each of which con- 

 tains both thick and thin fibers. In the tectum most of these fibers 

 end by wide arborizations in a common pool of intermediate neuropil 

 (fig. 93, layer 2). 



TECTUM OPTICUM 



In embryological development the first afferent fibers to reach the 

 tectum are those of the optic tract. These terminate in its anterior 

 part near the posterior commissure, and tectal structure matures in 

 subsequent stages from this region posteriorly. In the adult the com- 

 mon pool of neuropil, to which reference was made above, also re- 

 ceives similar terminals of the spinal and bulbar lemnisci, the bra- 

 chium of the superior colliculus from the thalamus, the strio-tectal 

 and habenulo-tectal tracts, and some other fibers. This pool of neu- 

 ropil is spread throughout the entire tectum, with little evidence of 

 localization of function within it. 



The optic tectum is structurally nearly homogeneous, with two 

 exceptions: (1) a dorsal thickening on each side, which is related 

 with the dorsal tectal commissure and probably corresponds (in some 

 respects only) with the torus longitudinalis of fishes ('42, pp. 250, 

 287), and (2) the eminence of the posterior commissure at the an- 

 terior border of the tectum, likewise chiefly commissural in function. 

 The stratification of the tectum, which is so conspicuous in the frog 

 and many other vertebrates, is here notably absent. The only layers 



