MOTOR NUCLEI IN PHYLOGENY 89 



of the selachian arrangement of the VII-IX-X visceral motor 

 nucleus appeared to be an indication of the relative importance 

 of the respiratory function of the hyobranchial musculature 

 (5, p. 412). Similarly in reptiles it would seem that the central 

 association of glossopharyngeal with facial motor perikaryons 

 rather than with vagal elements may be due to a rearrangement 

 of the visceral motor nuclear pattern largely as a consequence 

 of the loss of the hyobranchial pump mechanism for pulmonary 

 ventilation, and, as Kappers has shown (30, 32, 33), under direct 

 influence of the caudal VII-IX taste center. 



Motor V nucleus and root. With regard to the arrangement 

 of the elements of the motor V nucleus the reptiles examined are 

 evidently divisible into two distinct groups, to the first of which 

 belong Damonia, Chelone, and Alligator and to the second Boa 

 and Varanus. 



In the first group the motor V nucleus is for the most part 

 dorsally situated, but the cells in its caudal portion show a 

 tendency toward ventral (peripheral) displacement, a process 

 which in Alligator has resulted in the formation of a distinct 

 caudo ventral moiety of the nucleus (figs. 11, C, 11, D, and 12, A.) 



It has been shown that in its primitive position the motor 



V nucleus in ichthyopsidans occupies a dorsal position on or 

 near the level of its root exit (4, 5). Among reptiles the motor 



V nucleus has retained this primitive position to a large extent 

 in those forms in which the differentiation of the jaw musculature 

 is least advanced, viz., chelonians (Adams, 1, p. 89 et seq.). 



Evidence of a greater degree of nuclear differentiation is to 

 be seen in Alligator as noted above. It is significant, in view of 

 this, that the m. capiti-mandibularis in this form in contrast 

 to chelonians is further differentiated to form superficial, middle, 

 and deep muscles, while the m. pterj^goideus anterior is supple- 

 mented by the presence of a m. pterygoideus posterior not differ- 

 entiated in chelonians (Adams, 1. c). 



The distinction between the type of V nuclear arrangement 

 obtaining in Alligator and chelonians is not fundamental, but 

 one of degree only, and there can be little doiibt that this is to be 

 correlated with the essential similarity of their V muscular 



