292 George L. Streeter 



The n. oculomolorius arises from a group of neuroblasts forming the 

 ventral part of the mantle layer in the mesencephalon. These neuro- 

 blasts converge to form small rootlets which pass through the ground 

 bundle and emerge on the ventral surface of the neural tube in the con- 

 cavity of the cephalic bend. Here they unite into a common trunk, 

 which at first passes directly ventralward and then making a slight angle 

 bends lateralward and finally breaks up median and between the first 

 and second divisions of the trigeminal nerve, in the cellular mass which 

 is to form the eye muscles. 



The n. troclilearis arises from a group of neuroblasts similar and 

 lying just caudal to those of the oculomotor. The rootlets derived from 

 them, instead of emerging directly ventralward, curve dorsalward to 

 reach the roof of the isthmus, where they decussate and emerge as a 

 solid trunk which passes down lateral to the neural tube and breaks up 

 among the cells which are to form the superior oblique muscle. 



The n. abducens arises from a group of neuroblasts forming the 

 median part of the mantle layer directly beneath the fourth rhombic 

 groove. See Fig. 1. The rootlets arising from these neuroblasts pass 

 directly ventralward and after emerging they unite to form the main 

 trunk, which bends forward immediately at an angle of nearly 90°, and 

 passes forward into the region of the terminal rootlets of the oculo- 

 motor, as shown in Plate I. The relative positions of the abducens and 

 the facial nerve will be referred to under the description of the latter 

 nerve. 



Visceral Group. 



The facial, glossopharyngeal and vagus form a series of similar nerves 

 which consist almost wholly of visceral fibres. As can be seen on Plates 

 II and III, the visceral motor fibres arise from the nucleus ambiguus, 

 which consists of a column of neuroblasts continuous with the lateral 

 horn cells of the cord. The visceral sensory fibres arise from the periph- 

 eral ganglia and enter the alar plate of the neural tube and form a 

 longitudinal strand which in the adult we know as the tractus solitarius. 

 In addition to these visceral fibres there is a small number of somatic 

 sensory fibres, supplying the integument of the adjoining region, which 

 arise and have a course similar to the visceral sensory fibres. In aquatic 

 vertebrates there are also the special somatic sensory fibres of the lateral 

 line system, whose fibres join the rootlets of the facial, glossopharyngeal 

 and vagus to reach the brain, and the ganglia from which these fibres 

 are derived become incorporated in the geniculate, petrosal and nodosal 



