364 THE BRAIN OF THE TIGER SALAMANDER 



isthmic and bulbar tegmentum. All cell bodies are imbedded in this dense fibrillar reticulum. On 

 these small-scale drawings it is impossible to portray the delicacy and complexity of this tissue, 

 and the attempt is made to indicate the general trend of its coarser fibers. 



Fig. 79. — This section is median in the floor of the ventricle from the fovea isthmi to the 

 level of the IX nerve roots. X 40. The ventricular floor from the fovea to the tuberculum 

 posterius is projected from two adjacent sections, as indicated by a dotted line. This section is 

 very thick and is so inclined dorsoventrally as to include above the ventricular floor a thin 

 slice of the nucleus of the tubercidum posterius, tegmentum isthmi, and rostral end of the 

 trigeminal tegmentum of one side, ^'entrally of the ventricular floor in the isthmus region the 

 section cuts the interpeduncular nucleus and neuropil close to the mid-plane on the opposite 

 side. The level of the VIII nerve roots is indicated by the decussation of Mauthner's fibers, 

 from which one of the fibers is seen descending. The other fiber appears in the adjacent section 

 24. From the latter section tlie portion of tr. interpedunculo-bulbaris dorsalis (ir.inp.b.d.) 

 spinalward of this decussation is added to the drawing, showing that some fibers of this tract 

 descend as far as the level of the IX nerve roots. None of its fibers have been seen to extend 

 farther spinalward. The ventral division of this tract {Ir.inp.h.r.), on the contrary, goes much 

 farther, perhaps as far as the spinal cord. 



At the fovea isthmi several ependymal elements are impregnated, and a small artery here 

 enters the foveal pit at the ventral surface. These ependymal elements are crossed by unim- 

 pregnated myelinated fibers of the ventral tegmental fascicles {f.m.f.{l)), as indicated by dashed 

 lines. A single small neuron of the nucleus of the tuberculum posterius is impregnated. Its 

 dendrites spread downward among slender tortuous axons of tr. mamillo-peduncularis, which 

 form a dense neuropil in the gray of the peduncle. In this plane no fibers of tr. mamillo-inter- 

 peduncularis are stained, but they are abundant farther laterally. 



Unimpregnated cell bodies of the interpeduncular nucleus are clearly visible, arranged as 

 indicated by the dotted outlines. Among these and ventrally of them is dense neuropil, very 

 inadequately shown in the drawing. These fibers are derived in part from tr. mamillo-inter- 

 peduncularis and from the overlying tegmentum, and in larger part they are axons of the cells 

 of the interpeduncular nucleus. These axons take tortuous courses, mainly directed dorsalward 

 into the isthmic and trigeminal tegmentum and spinalward into the dorsal and ventral inter- 

 pedunculo-bulbar tracts. From them arise numberless collaterals which ramify in the inter- 

 peduncular neuropil and enter the glomeruli (fig. 83). 



Fig. 80.— The adjoining section. X 75. The ependymal floor of the ventricle is slightly to one 

 side of the mid-plane, ventrally of which the section is inclined laterally. Heavy broken lines 

 mark the upper and lower limits of the zone of unimpregnated cell bodies of the interpeduncular 

 nucleus. Scattered cells of this nucleus are distributed in the underlying neuropil except its 

 ventral part. Above this zone of den.sely crowded cells are the less crowded cells of the isthmic 

 and trigeminal tegmentum, none of which are impregnated. One neuron of the interpeduncular 

 nucleus is impregnated with dendrite directed ventrally into the neuropil. Fragments of other 

 dendrites, with tufted terminals in glomeruli, are spread throughout tlie neuropil, some of 

 which are drawn (compare figs. 65, 66, 81-84). There is no other impregnation in the ventral 

 interpeduncular neuropil containing the unimpregnated spiral endings of the f. retroflexus, 

 but dorsally of this there are stained axons of various sorts among the dendrites. These are not 

 drawn, except those lying between the cells of the interpeduncular nucleus and the myelinated 

 fibers of the ventral commissure, the most dorsal bundles of which are outlined with dotted 

 lines. Most of the.se immyelinated fibers enter tr. interpeduncvdo-bulbaris dorsalis. The inter- 

 peduncular neuropil is broadly connected with the overlyiiig tegmental neuropil by fibers 

 passing in both directions. In this plane (not drawn) there are a few fibers of tr. niamillo-inter- 

 peduncularis and more of them farther laterally. 



Figure ,Si .—Obliquely sagittal Golgi section of an adult brain taken close to the mid-plane. 

 X 60. Some other sections of this specimen have been published (fig. 101; '25, figs. 14, 40). 

 The plane of section of this figure is similar to that of figure 79, but somewhat more oblique. 

 The ventral surface is nearly median in the isthmus, and dorsally and anteriorly of this the 

 plane is inclined laterally. Since the sections are very thick, the dorsal ventricular border of the 

 tuberculum posterius (indicated by the dotted line), and a slice of the lateral wall dorsally of it 

 are included in the section outlined. The detail is drawn from this section and the adjacent 

 section laterally. The four components of the commissure of the tuberculum posterius (p. 302) 



