ILLUSTRATIONS 



345 



Fig. 38. — This section on the left cuts the superficial origins of the V, VII, and VIII nerve 

 roots and, farther forward, of the III root. The sensory V fibers are seen to bifurcate with more 

 slender ascending branches, most of which arborize in a neuropil, wliich is the primordiuni of 

 the superior sensory V nucleus of mammals. The prefacial f. solitarius fibers are smooth and 

 unbranched to their terminals rostrally and internally of the entering V root. Some impreg- 

 nated fibers of the vestibular root (r. VIII.) extend forward to end within and rostrally of the 

 superior V neuropil, but these are not drawn. Internally of tlie trigeminal neuropil and conflu- 

 ent with it are terminals of ascending crossed and uncrossed fibers termed tr. bulbo-isthmialis 

 (fr.b.is.), which is continuous with tr. spiuo-bulbaris {tr.sp.b.). These fibers ascend and descend 



tr.b.sp.+tr.b.is 



tr.sp.b. 

 tr. b. sp. 



from all levels of the medulla oblongata, many of them first decussating as internal or external 

 arcuate fibers. Many of them bifurcate into ascending and descending branches with or without 

 decussation. They accompany the spinal lemniscus and comprise a mixed bulbo-spinal and 

 spino-bulbar system. 



Fig. 39. — This section, parallel with the ventral surface, illustrates the courses of some of 

 the external arcuate fibers, many of which are impregnated on the right side and none on 

 the left. Useful landmarks are provided by the decussation of Mauthner's fibers (fib.M.) and 

 two blood vessels, which, farther dorsally, are related with the roots of the V (b.r.V.) and VII 

 (b.r.VlI.) nerves. No fibers of lemniscus and secondary visceral systems are impregnated. 

 The visible fibers evidently are concerned chiefly with bulbar and spinal adjustments. 



Figure .1,0. — Detail from a horizontal Golgi section of a late larva, showing the entrance of 

 the sensory V root and three neurons of the motor V nucleus. X 50. The dendrites extend 

 laterally to engage collaterals of the sensory root fibers, and in the adjoining section ventrally 

 other dendrites of these cells ramify downward into the ventral alba, engaging collaterals and 



