218 THE BRAIN OF THE TIGER SALAMANDER 



the vicinity of the nucleus of the III nerve (fig. 59) have the chief 

 dendrites directed rostrad, some of them longer and more sharply 

 bent forward than those shown in figure 59. These dendrites engage 

 fibers from the basal optic tract, visceral-gustatory system, hy- 

 pothalamus, tr. olfacto-peduncularis, and mesencephalic terminals 

 of the f. retroflexus, among other systems. Their axons also are di- 

 rected forward; how far they extend within the basal forebrain 

 bundles is not evident, probably for a long distance because they are 

 thick, smooth fibers similar to those of the long descending and 

 ascending tracts. It is not improbable that they reach the primordial 

 corpus striatum, where similar fibers end in wide terminal arboriza- 

 tions ('396, figs. 39, 41, 50, 72, 73; '42, fig. 69). These fibers may be 

 comparable with those described in mammals as ascending from the 

 entopeduncular nucleus and substantia nigra to the corpus striatum. 



The neuropil of the alba resembles that of the ventral thalamus, 

 but that of the gray substance is more dense and of different com- 

 position. In the white substance there are mingled terminals of axons 

 derived from a great variety of sources — tectum, basal optic tract, 

 posterior commissure and its nucleus, dorsal and isthmic tegmentum, 

 the entire diencephalon, and the basal forebrain bundles (figs. 12, 14, 

 15, 17, 18, 20, 21, 22). There is little evidence of regional localization 

 of these terminals except in two places. 



Posteriorly, in the vicinity of the nucleus of the III nerve, the deep 

 neuropil of the gray is very dense, with strands of fibers de- 

 scending from the tectum and the dorsal and isthmic tegmentum 

 (fig. 24). These clearly envelop the cell bodies of the III nucleus, and 

 many of them decussate here in the ventral commissure. A second 

 and more sharply localized area of neuropil is the area ventrolateralis 

 pedunculi, extending superficially from the III root forward (fig. 23), 

 as described in chapter iii. 



The commissure of the tuberculum posterius includes all fibers of 

 the ventral series of commissures which cross in the midbrain; for 

 details of its connections see chapter xxi. 



Summary. — The peduncle of Amblystoma as here defined receives 

 fibers from practically all parts of the brain above the isthmus, and 

 its primary function is control of mass movements of the trunk and 

 limbs and conjugate movements of the eyes. Control of local reflexes 

 is effected elsewhere. It is intimately connected with the hypothala- 

 mus, and various visceral motor adjustments are made here, though 

 of these little is known. 



