MOTOR NUCLEI IN PHYLOGENY 93 



anurans). In these forms, in contrast to gill-breathing ichthy- 

 opsidans, the eye when at rest is normally focused for distance, 

 and it has been suggested that the development of the some- 

 what specialized oculomotor nucleus observed in Rana was 

 probably to be correlated with the acquisition of this type of 

 visual apparatus (5). 



In most reptiles as in anurans the eye at rest is focused for 

 distance, i.e., parallel rays are brought to focus on the retina 

 without accommodation effort. In reptiles, with the exception 

 of snakes, the mechanism for accommodation has become highly 

 elaborated through the development of a striate ciliary muscula-" 

 ture. With the foregoing exception the striate ciliary musculature 

 is well developed in all reptiles and especially so in chelonians 

 (Wiedersheim, 44, p. 282). l&y the action of this ciliary muscle 

 on the lens capsule the curvature of the latter may be altered 

 and accommodation effected as in mammals. In snakes the 

 mechanism for accommodation resembles that obtaining in 

 amphibians where the lens itself is shifted in its entirety. 



The m. sphincter iridis in most reptiles is composed of striate 

 muscle fibers, so that in general these forms, with the possible 

 exception of snakes, may be said to possess a highly developed 

 and complex intrinsic ocular effector mechanism. 



From the above observations it might be concluded that the 

 relatively slight differentiation of the oculomotor nucleus in Boa 

 is to be correlated with the absence of the striate ciliary mechan- 

 ism in this form. To some extent no doubt this may be true, 

 but certainly no such explanation can be offered for the slight 

 amount of oculomotor nuclear differentiation observed in Chelone 

 and Damonia, nor does it appear that the ciliary mechanism is 

 more highly developed in Alligator and Varanus in correspondence 

 with the evident oculomotor nuclear specialization of those forms. 



Alligator and Chamaeleon resemble one another and differ 

 in degree at least from the other reptiles examined in one respect, 

 viz., in the presence in the retina of a recognizable fovea centralis 

 or point of maximum visual acuity (Slonaker, 45). It is highly 

 probable that in Varanus a similar condition obtains. The 

 fovea centralis is especially well developed in Chamaeleon, there 



THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 32, NO. 1 



