CRANIAT. NERVES 141 



rected posteroventrally in loosely arranged dorsal and ventral fas- 

 cicles, which converge toward the V nerve roots. 



The ovoid cell body has a smooth contour, with no processes except 

 the single thick fiber. It is imbedded in dense neuropil and closely 

 enveloped by a web of these fibers. Every contact of the fibers of the 

 neuropil with the cell is a synaptic junction. This is doubtless the 

 explanation of the wide dispersal of these cells in all parts of the 

 tectal gray, and they are so arranged that the entire extent of the 

 deep tectal neuropil may be simultaneously activated by excitation 

 of the mesencephalic V system. 



The striking resemblance of the cells of the mesencephalic V 

 nucleus with those of the semilunar and spinal ganglia and with the 

 transitory Rohon-Beard cells of the spinal cord (Coghill, '14, Paper 

 I) has often been commented upon and is well illustrated by the 

 excellent photographs pul)lished by Piatt. They have, accordingly, 

 been generally regaixled as derivatives of the embryonic neural crest 

 that have remained within the neural tube. This hypothesis has been 

 tested experimentally by Piatt ('45), with the conclusion that neural 

 crest is at least one source of these cells, though a possible origin from 

 other sources is not excluded. Many observers have reported the 

 presence of intramedullary cells of sensory type along the courses of 

 roots of spinal and cranial nerves (Pearson, '45, cites instances), and 

 some of these cells also may be of neural-crest origin. Others may be 

 of autonomic type, migrating out from the brain (Jones, '45), though 

 this is controverted. 



Just as the typical unipolar cells of the sensory ganglia of spinal 

 and cranial nerves have a single process, which divides into periph- 

 erally and centrally directed branches, so the mesencephalic V fibers 

 (or some of them) divide shortly before emergence from the brain 

 into peripheral and central branches (fig. 13). The central branches 

 descend as far as the level of the IX nerve roots. My earlier statement 

 ('14a, p. 362) that these fibers ."arborize among the dendrites of the 

 motor VII neurons" is misleading, for these terminals are spread 

 widely in the intermediate zone between the levels of the V and the 

 IX roots. 



This bifurcation of the root fibers and the fact that the bodies of 

 the cells of the mesencephalic V nucleus are in synaptic contact with 

 all the deep neuropil of the tectum suggest that afferent impulses 

 transmitted by these fibers may take either or both of two courses: 

 (1) They may pass upward to the tectum, where they activate the 



