MOTOR NUCLEI IN PHYLOGENY 91 



3. Eye-muscle nerves 



Nerve VI. In the descriptive portion of this paper it was 

 noted that the number of the emergent abducens rootlets and 

 the level of their exit from the brain stem in reptiles was subject 

 to considerable variation. Corresponding variations in the 

 relative position of the abducens nucleus are also to be observed 

 in reptiles, and in this respect they afford a striking contrast to 

 the condition obtaining in this area among amphibians. 



It has been pointed out that in its primitive position the ab- 

 ducens nucleus is dorsally placed in intimate contact with the 

 fasciculus longitudinaUs medialis and lies caudad of the exit 

 level of the motor VII root. In this location it is found in 

 selachians, ganoids, dipnoans and amphibians, (figs. 10 and 11). 



In Damonia the abducens nucleus and roots retain their 

 primitive position, but in Chelone the nucleus is elongated some- 

 what in a rostral direction and some of the VI rootlets emerge 

 on a level with the motor VII root. A similar condition obtains 

 in Alligator, but in the more recent Squamata (Boa and Varanus) 

 all the abducens rootlets emerge rostrad of the motor VII and 

 the abducens nucleus as a whole has shifted rostrad. 



In reptiles the ventral tectobulbar fiber sj^stem is of but small 

 size while the dorsal tectobulbar fibers are considerably developed 

 (de Lange, 11). Further, the vestibular nuclei become for the 

 first time in phylogeny considerably elaborated in reptiles 

 (Holmes, 27). Finally, Kappers has demonstrated that a con- 

 stant relationship may be observed between Deiters' nucleus 

 and the rostral end of the abducens nucleus. 



On the basis of these facts Kappers has shown that the position 

 and morphology of the abducens nucleus in the different reptilian 

 forms examined may be directly correlated with the rise in 

 importance of the dorsal tectobulbar tract and vestibular system 

 within the brain stem of these animals.^ 



In Boa and Varanus an abducens root and nuclear pattern 

 obtains which resembles that constantly found in many mammals 



^ For a full discussion of this subject reference should be had to Kappers' 

 earlier papers, especially 31 and 32. 



