306 THE BRAIN OF THE TIGER SALAMANDER 



and the dorsal fascicles of the ventral commissure posteriorly of the fovea isthmi are outlined 

 with dotted lines. The decussation of the anterior division of tr. tecto-bulbaris cruciatus in the 

 ventral median tegmental fascicles {J.m.t.{l)) is indicated by dashed lines (compare '36, fig. 2). 



A neuron of the dorsal (mamillary) part of the hypothalamus and the dendrite of another 

 are impregnated. Their axons branch, and one branchlet enters tr. mamillo-interpeduncularis. 

 This tract partially decussates in component 1 of the commissure of the tuberculum posterius 

 (the retroinfundibular commissure of the literature), and its slender unmyelinated fibers ar- 

 borize in the rostral part of the interpeduncular neuropil chiefly laterally of the plane here 

 shown (compare '36, figs. 3, 8, 20). Two ependymal elements are drawn. These lie near the 

 ventral median raphe under the cerebellum (compare figs. 63, 64, 70). 



Unimpregnated cell bodies of the interpeduncular nucleus are scattered under the ependy- 

 mal surface and among the myelinated median fascicles and ventral commissure bundles. A 

 well-impregnated dendrite of one of these is drawn; it has a tufted terminal near the ventral 

 surface (compare figs. 62, 65, 66, 80, 82). There is scanty and very clear impregnation of some 

 of the coarser fibers of the interpeduncular neuropil. Those in its anterior part are derived chief- 

 ly from tr. mamillo-interpeduncularis and probably some from tr. olfacto-peduncularis, which 

 is partially impregnated more laterally (fig. 101). More posteriorly the visible fibers all seem to 

 be axons of cells of the interpeduncular nucleus which assemble to descend in the dorsal and 

 ventral interpedunculo-bulbar tracts. 



Figure 82.— k semidiagrammatic drawing of a thick Golgi section from an obliquely sagittal 

 series of an adult brain. X 37. The section is nearly median posteriorly (at the right), and 

 anteriorly it is much more lateral, including the f. retroflexus in the peduncle. Some fibers of 

 this fa.sciculus descend uncrossed into the interpeduncular neuropil. Mingled with the un- 

 cros.sed fibers of the f. retroflexus are thicker and smoother fibers of tr. olfacto-peduncularis, 

 some of which enter the interpeduncular neuropil. Dorsally of these is the mamillo-peduncular 

 tract, comprising ventral tegmental fascicle (3), and still farther dorsally are fibers of tegmental 

 fascicle (6), which come from the postoptic commissure. These tegmental fascicles contain both 

 myelinated and unmyelinated fibers, and both sorts are here darkened by the Golgi treatment. 

 Between the two tegmental fascicles just mentioned are the darkened thick myelinated fibers 

 of the anterior division of tr. tecto-bulbaris cruciatus (f.m.1.{l)), some of which decussate near 

 the fovea isthmi dorsally of the olfacto-peduncular fibers and others descend in the ventral 

 median fascicles. One neuron of the interpeduncular nucleus is well impregnated and dendrites 

 of several others, all of which exhibit the characteristic dendritic tufts. No ependyma, spiral 

 fibers, or axonic tufts are impregnated in the interpeduncular neuropil. The scanty axonic 

 neuropil is probably composed exclusively of terminals of uncrossed fibers of the f. retroflexus 

 and olfacto-peduncular tract. 



Figure SJ.— Diagram of a neuron of the interpeduncular nucleus, seen as projected upon 

 the median sagittal section. X 40. This is a composite drawing from observations made on 

 many sections cut in various planes. Compare figures 19 and 84. 



Figure 84. — Diagram of the composition of the interpeduncular glomeruli. X 45. This 

 drawing, like figure 79, shows the median section of the floor plate and dorsally of this an 

 oblique slice of the overlying tegmentum. Two neurons of the interpeduncular nucleus are 

 drawn. Tufted collaterals from the axon of the anterior element engage dendritic glomeruli of 

 the posterior element. Tufted axonic terminals enter glomeruli from small cells of the isthmic 

 and trigeminal tegmentum and also from collaterals of tr. tegmento-bulbaris, which arises from 

 large cells of the same tegmental areas. Compare figures 19 and 83. 



Figure 85. — The lateral aspect of the adult brain, drawn from the same wax model as figure 

 lA. X 15. This drawing shows the courses of the four most superficial components of the stria 

 medullaris thalami seen as projected upon the lateral surface (fig. 20 and chap, xviii). Tractus 

 olfacto-habenularis lateralis (2) goes directly dorsally from the preoptic nucleus. Tractus 

 olfacto-habenularis anterior arises in ventral (3) and dorsal (4) divisions from the anterior 

 olfactory nucleus. Tractus cortico-habenularis lateralis (5) arises from the primordial piriform 

 lobe and joins no. 4 before entering the stria medullaris. 



