24 EVOLUTION OF THE HUMAN EYE 



of the movements of the eye in the interests of binocular 

 vision. 



Some mammals have the power of markedly increasing 

 the prominence of the eyes in the orbit, and so temporarily 

 of increasing the extent of their monocular fields. In 

 burrowing animals, such as moles and hedgehogs, the eyes 

 are retracted, for the sake of protection, when they are 

 below ground, and are protruded, beyond their epidermal 

 coverings, when they come to the surface. In other mammals 

 increased prominence of the eyes becomes most noticeable 

 when they are alarmed, and when increased circumferential 

 vision is most likely to be of importance to them. The 

 increase in the prominence of the eyes is effected by relax- 

 ation of the retractor bulbi muscle, and contraction of the 

 muscle of Gegenbauer, which lines the fibro-elastic tissue 

 rilling in the outer wall of the orbit in animals where it is 

 not bounded by bone. A vestige of this muscle is some- 

 times to be found in man opposite the sphenomaxillary 

 fissure, and is termed Miiller's muscle. I shall have more 

 to say respecting it later, in speaking of the protective 

 mechanisms of the eyeball. I refer to it here because slight 

 protrusion . of the eyes in human beings, as the result of 

 intense emotional excitement, has been observed; though 

 "starting of the eyes from the head" as the result of fright 

 is more often read of in fiction than actually seen in real 

 life. 



An increase in the size of the cornea relatively to the size 

 of the eyeball must allow rays of light of greater obliquity 

 to be refracted into the eye, and an increase of the corneal 

 surface tends to increase not only the size of the field of 

 vision, but also the clearness of peripheral images. 



In some rodents the cornea composes nearly half the 

 surface of the eyeball. The relations between the diameters 



