360 



THE BRAIN OF THE TIGER SALAMANDER 



a thick broken line. In the posterior hp of this fissure there is massive elective impregnation of 

 the brachium conjunctivura and the locus of the decussation of these fibers more medially is 

 marked by three crosses. Before and after crossing, many of these fibers turn spinalvvard in 

 the superficial alba of the isthmic tegmentum. 



Figures 72 and 7S. — Two semidiagrammatic drawings from an obliquely longitudinal series 

 of Golgi sections of an adult brain. The sections are cut at an angle of about 30° from the 

 sagittal plane, with dorsal and posterior sides more lateral. 



Fig. 72. — These details are assembled from several consecutive sections to illustrate some 

 components of the complex of fibers at the di-telencephalic boundary. X 25. Only some of 

 the thicker unmyelinated axons are impregnated in this preparation. The ventral border at the 



left is near the median plane, and posteriorly of this place the section is inclined laterally. At 

 the posteroventral border of the primordium hippocampi some thick fibers are seen to converge 

 at the thalamic junction, and Weigert sections show that a few of them are myelinated. The larg- 

 er number of these fibers descend to the hippocampal commissure, and a smaller number a.scend 

 in the stria medullaris thalami as tr. cortico-habenularis medialis. Others descend obliquely pos- 

 teroventrad into the ventral thalamus, passing through and posteriorly of the gray of the 

 eminentia thalami^ — tr. cortico-thalamicus medialis. This is the precursor of the mammalian 

 columna fornicis, but here apparently none of these fibers reach the mamillary region of the 

 hypothalamus. These thick fibers are accompanied by far more numerous very thin fibers, 

 some of which arise as collaterals from the decussating fibers of the hippocampal commissure 

 (fig. 71). An impregnated neuron of the bed-nucleus of the anterior commissure ridge is 

 sketched from the most lateral of the sections here represented. Its dendrites spread dorsally 

 and ventrally among the hippocampal fibers; the course of the axon is not revealed. 



The well-impregnated fibers of tr. thalamo-frontalis arise from the cells of the dorsal 

 thalamus shown in figure 73. In the chiasma ridge, chiasmatic bundles 2 and 3 of optic fibers 

 are impregnated, and their terminals in the lateral part of the tectum are seen at the top of 



