696 



THE EYE IN EVOLUTION 



Cormorant 



pair of pliers. To a large extent this immobility is compensated, as in 

 snakes, by the extreme mobility of the neck, the constant agitated 

 swivelling and nodding movements of the head continually varying 

 the visual fields ; the hawk, for example, can swivel its head around 

 through an angle of 180°, the owl, of 270°. Moving objects are thus 

 followed by movements of the head and gross ocular reflex movements 

 are taken over by neck movements. The presence of two fovese also 

 lessens the need for movements of the eyes while the nasal asymmetry 

 of the eye and its dioptric elements supply the amount of convergence 

 necessary for the binocular function of the temporal foveas so that 



convergent movements for this purpose 

 are not usually required. It is true that 

 limited ocular movements are seen in 

 some species, particularly the parrots, 

 large-beaked birds such as the hornbills 

 or the toucan, and aquatic birds such as 

 the crane, the seagull, the penguins and 

 the cormorant (Rochon-Duvigneaud, 

 1943). In these, however, movements 

 of the two eyes are always dissociated 

 except for movements in the horizontal 

 field in the interests of convergence. 



MAMMALS are unique in their ocular 

 movements, whether lateral, vertical, 

 oblique or convergent, in so far as they 

 are always conjugated. In the lower 

 classes, however — Insectivores, Cliirop- 

 tera, Edentates and Rodents — move- 

 ments are very restricted, if, indeed, 

 they exist ; even visually conscious 

 Rodents such as the squirrel and the marmot hardly move their 

 eyes although they are busily engaged in exploring space intently 

 all the while, doubtless because in their cone-rich retina visual 

 acuity is everywhere excellent (Rochon-Duvigneaud, 1943) ; con- 

 versely in the mouse or the rat the eyes are kept motionless, probably 

 because vision is everywhere so poor (Walls, 1942). In Ungulates 

 ocular movements are more conspicuous ; the eyes of the elephant, 

 however, are relatively immobile despite the enormous size of the 

 extra-ocular musculature. ^ In the larger Carnivores of the cat and 

 dog families, they are freer ; and in the foveate higher Primates 

 they are most conspicuous of all. In all cases, however, as in man, they 

 are largely supplemented by movements of the neck, and the head is 



1 p. 497. 



Fig. 846. — A Drawing of Tarsius. 



Looking directly behind itself ; 

 to show the extraordinary mobility 

 of the neck to compensate for the 

 immobility of the eyes. 



