MOTOR NUCLEI IN PHYLOGENY 71 



arcuatae dorsales. In this dorsal situation the nucleus extends 

 from a short distance behind the caudal border of the emergent 

 motor VII root to a point slightly rostrad of the exit level of 

 the IX motor root. The abducens fibers course ventrad and 

 slightly laterad to reach the periphery, where they emerge as 

 three fine rootlets in linear series between the exit levels of the 

 motor VII and IX roots (figs. 4 and 11, C). 



A comparison of the reconstruction charts B and C in figure 

 11 shows that a striking similarity exists in the topographical 

 relations of the abducens roots and nucleus in Damonia and 

 Rana catesbiana. 



In reptiles, as Kappers has pointed out, the abducens nerve 

 is subject to considerable variation both in the number and 

 place of exit of its roots as well as in the topographical relations 

 of its nucleus. Thus in Alligator eleven rootlets are present, 

 three of which emerge caudad of the motor IX root ; in Chelone 

 six rootlets are present, the most caudal of which emerges at the 

 exit level of the motor IX root; in Damonia six rootlets are 

 present as described above; in Anolis carolinensis several rootlets, 

 the number of which is not given, emerge at the same relative 

 level as in Damonia (Willard, 1. c, p. 45 and pi. 2, fig. 4) ; in Boa 

 the six rootlets all emerge rostral of the exit level of the motor 

 VII nerve; in Varanus five rootlets are present, of which the first 

 three are very close together and the last two are very small, all 

 of them emerging rostrad to the exit level of the motor VII 

 root. The corresponding variations in nuclear topography will 

 be described in connection with the subsequent discussion. 



In Hatteria the abducens nucleus consists of a somewhat 

 scattered cell group lying lateral and dorsal to the fasciculus 

 longitudinalis mediahs (Gisi, 20, p. 178). Its emergent fibers 

 form four or five rootlets upon the ventral surface of the brain 

 stem, but Gisi does not note their relation to the exit level of 

 the motor VII root. 



