358 THE BRAIN OF THE TIGER SALAMANDER 



electively stained, with tufted endings in the interpeduncular neuropil similar to those shown 

 in figures 61 and 6:2. At the level of figure 65 only a few of these fibers are impregnated and 

 also a few neurons of the interpeduncular nucleus with dendrites extending downward into the 

 interpeduncular neuropil. 



In figure 66 the neurons of the interpeduncular nucleus are seen to have thick dendrites 

 extending laterally to ramify widely in the alba of the isthmic and trigeminal tegmentum and 

 thinner dendrites directed ventrally into the interpeduncular neuropil. Both dendritic and 

 axonic arborizations enter the glomerulus-like tufts; but, in order to clarify their relations, 

 only the dendritic component is drawn on the left side and the axonic component on the 

 right (compare figs. 83, 84). 



Figure 67.- — Transverse section through the middle of the interpeduncular nucleus of a half- 

 grown larva. X 75. The broken line marks the outline of the gray substance. The Golgi im- 

 pregnation is scanty, showing in this region only a few decussating fibers of tr. tegmento-bul- 

 baris on a clear ground. The more dorsal of the two fibers shown is sketched from the adjoining 

 section spinalward. These thick axons probably arise from unimpregnated cells of the isthmic 

 tegmentum, as shown in figiu-e 68. In the ventral commissure, where these fibers decussate, 

 slender collaterals leave them to ramify in the interpeduncular nucleus and its neuropil. 



Figure 6S. — A diagram based on Golgi sections of a larval Arablystoma from the same lot 

 as figure 67. X 75. The sections are obliquely transverse. The section outlined is in about the 

 same plane as figure 67 on the right and passes through the auricle on the left. The details are 

 from this and the two adjoining sections. Everything drawn was observed, but the assembly is 

 schematic. Thick axons of neurons of the isthmic tegmentum {tr.feg.b.) converge into the ven- 

 tral commissure, where slender collaterals separate from them to arborize in tufted form in the 

 interpeduncular neuropil. 



Figures 69 a/id 70. — These two drawings illustrate details of the interpeduncular region at 

 and immediately rostrally of the nucleus of the IV nerve (compare the diagram, fig. 68). 

 X 50. The impregnation of the transverse Golgi sections of this adult brain is exceptionally 

 good, and it has been quite fidly illustrated (';*7. figs. ^4-30; U, figs. 24-31). 



Fig. 69. — This section is adjacent posteriorly to the one shown in figure 30 of 1942, to which 

 reference may be made for the topographic relations. In both figures the bundles of myelinated 

 fibers of the ventral and ventromedian tegmental fascicles are outlined with broken lines. In 

 the interpeduncular region of the section here shown, there is no impregnation of any nervous 

 elements except a few fibers of tr. tegmento-bulbaris at their decussation in the ventral com- 

 missure. Within the commissure slender collaterals descend into the interpeduncular neuropil, 

 where some of them have tufted endings. 



Fig. 70.^This is drawn from the second section spinalward of figure 69, through the nucleus 

 of the IV nerve, one neuron of which is impregnated. Thick myelinated axons of the IV nerve 

 root (darkened by the Golgi fluid) ascend from this nucleus in the position indicated by broken 

 lines. Typical ependymal elements of the ventral raphe are drawn. 



Figure 71. — Obliquely longitudinal section of the adult brain taken not far from the mid- 

 sagittal plane, with the dorsal and anterior sides inclined somewhat laterally, so that almost 

 the entire length of the f. retroflexus appears in a single thick Golgi section. Golgi method. 

 X 30. The drawing is semidiagrammatic, with some details added to the section outlined from 

 neighboring sections and from the opposite side. 



At the point where tr. cortico-habenularis medialis joins the stria medullaris, numberless 

 fine collaterals separate from it to enter the eminentia thalami. The section cuts the optic 

 nerve at its junction with the brain. Peripherally, each optic nerve between the foramen and 

 the brain shows several hundred impregnated unmyelinated fibers, most of which lose the stain 

 before entering the brain. The surviving impregnated fibers decussate in chiasmatic bundles 1 

 (ch.l.) at the extreme anteroventral angle of the chiasma and ascend toward the tectum as the 

 most rostral fibers of the marginal optic tract, losing the stain before reaching the tectum 

 ("42, p. 232). 



The abundant impregnation of the f . retroflexus extends only as far as the decussation, be- 

 low which its fibers are unstained. As these fibers turn medially toward the decussation, many 

 of them separate and descend uncrossed along the lateral margin of the interpeduncular neu- 

 ropil. The locus of the sidcus isthmi on the ventricular siu-face is projected upon the drawing as 



