134 THE BRAIN OF THE TIGER SALAMANDER 



sory V fibers are electively impregnated, often with no other fibers 

 visibk- in their vicinity. Some of these from the larva have })een ilhis- 

 trated ('14a, figs. 48-51, 54; '396, figs. 4'2, 46, 47, 57-61, 67, 77). 

 Figures 27-32 show V roots as seen in horizontal Cajal sections of the 

 adult brain. Figure 32 passes through the two motor V roots and 

 their nucleus; figure 40 includes three impregnated neurons of this 

 nucleus and the bifurcating fibers of the sensory root, which are also 

 shown in figure 38. Woodburne ('36, p. 451) saw thick root fibers of 

 the trigeminus entering the cerebellar root; but, in the absence of 

 Golgi sections, the thinner collaterals of the spinal root were not 

 demonstrable. 



The superior or cerebellar root of the trigeminus is much smaller 

 than the spinal root, and at the ventrolateral border of the auricle it 

 joins the spino-cerebellar tract (figs. 30, 31, 91). Many fibers of both 

 tracts end here with open arborizations in a neuropil which is the 

 primordium of the superior (chief, or pontile) nucleus of the mam- 

 malian trigeminus ; but some fibers of both tracts pass through this 

 neuropil and continue dorsomedially into the body of the cerebellum, 

 where they end, some on the same side and some decussating in the 

 commissura cerebelli (figs. 31-34, 37, 91). These commissural fibers 

 are joined by others arising from cells in the vicinity of the superior 

 trigeminal neuropil, and many of the decussating fibers, after cross- 

 ing, reach the superior neuropil of the other side, thus forming an 

 intertrigeminal commissure. We here confirm, in the adult, Larsell's 

 description ('32, p. 413) of the cerebellar commissure of the larva as 

 composed chiefly of trigeminal and spinal components. In Amblys- 

 toma the superior sensory nucleus of the trigeminus probably is con- 

 cerned chiefly with the proprioceptive aspects of cutaneous sensibil- 

 ity (deep proprioception being provided for in the mesencephalic V 

 root). This cerebellar connection persists in man, but here the chief 

 V nucleus has also acquired refined types of sensibility which Amblys- 

 toma lacks. 



Neither the superior nor the spinal nucleus of the trigeminus has 

 well-defined boundaries. The central cells which engage terminals and 

 collaterals of the sensory V fibers may also have synaptic contacts 

 with terminals of all other sensory systems that enter the medulla 

 oblongata. There are, however, certain lines of preferential discharge 

 for each group of sensory systems, and the segregation of local nuclei 

 and secondary pathways for each functional system is incipient. 



