EYE MOVEMENTS IN MAMMALS 311 



usually means horizontal. The hippopotamus, lying with the eyes just out 

 of water, is claimed by one author to be able to make even voluntary, 

 monocular, vertical movements like a sauropsidan. The modern hippo- 

 potamus has no aerial enemies, or indeed any known enemies at all; so 

 the value of this 'ability,' if it exists, is doubtful. It may be a necessity, 

 rather than an ability — imposed by the slender horizontal pupil. 



Where the angle of eye movement is small, the lid opening is also 

 small. One can thus judge the extent of ocular mobility in a given mam- 

 mal by noting how much of the white of the eye (the sclera) shows. 

 Spontaneous eye mobility is greatest in the higher primates, which alone 

 among mammals have a fovea; but even here it is supplemented to a sur- 

 prising degree by head movements, as we soon find out when we spend a 

 day with a stiff neck. It is next-best developed in the larger carnivores, 

 particularly the cat and dog families; but it is not so conspicuous in the 

 ungulates. It is probably only accidental that voluntary eye movement 

 seems best developed in the 'most intelligent' mammals, as has been 

 pointed out by some authors. The elephant, with high intelligence and 

 little eye movement, seems to be the exception which destroys the rule. 



The voluntary eye movements of mammals are really best correlated 

 with visual acuity, which, it so happens, does go pretty well with intelli- 

 gence in this group of vertebrates. The mammals obey the rule that such 

 movements occur only where there is a fovea or a circumscribed and dis- 

 tinct area centralis. Binocular employment of the two areas (in primates, 

 the two foveas) is so valuable and so constant that it has become fixed 

 in the neuromuscular apparatus as an unlearned habit, the expression of 

 which is the continuous conjugation of the two eyes — so different from 

 the mere temporary coordination of a fish or a chameleon. Again, the 

 urge toward binocular vision has operated in evolution to increase the 

 degree of frontality in the most eye-minded of mammals — the primates 

 and the larger carnivores and ungulates. In the individual mammal, even, 

 the urge to see binocularly is extremely powerful. Even in animals for 

 which it is a great labor, the head is turned to face squarely an object 

 which has taken the attention. Thus the horse, for example, fixates 

 objects binocularly until they approach within three or four feet, when 

 he is forced to turn his head away and continue his observations monoc- 

 ularly. In cats and dogs, if the insertions of the superior rectus and 

 external rectus muscles are surgically interchanged, or even if the ex- 

 ternal is removed and the superior brought down into its place, the eye 

 movements become completely re-conjugated in a few days. Recent work 



