40 EVOLUTION OF THE HUMAN EYE 



more than compensates man for the loss of the wide degree 

 of panoramic vision which the alteration of position of the 

 eyes from the side to the front of the head entails. For 

 these rapid changes of position of the spots with acute form- 

 sense, increased mobility, first, of the eyeballs, and secondly, 

 of the head, has been evolved. 



Though nearly all mammals have, as in man, four recti 

 and two oblique muscles connected with the eyeballs, the 

 movements of the globes in all those below monkeys are 

 comparatively slight. It is in association with the develop- 

 ment of acute central stereoscopic vision, as in monkeys, that 

 the movements of the eyes become a conspicuous feature. 

 The range of such movements becomes considerably extended 

 in man, and may, I suggest, be correlated with the full 

 assumption of the erect posture and the greater need for 

 a wide, quick circumferential vision on the ground than 

 in the trees. 



It is probably for the purpose of increasing the range of 

 movement of the eyes that man has developed a wider pal- 

 pebral fissure, in proportion to the width of the cornea, than 

 is met with in any other mammal. The effect of this pro- 

 portional difference is that a large part of the ocular con- 

 junctiva, with the sclerotic beneath, becomes exposed to 

 view r . In no other mammal are 'the whites of the eyes" 

 such a conspicuous feature as in man. An exposure of the 

 ocular conjunctiva in this way has rendered it liable to irri- 

 tation from atmospheric influences, more especially on the 

 inner side of the globe, where, due to the presence of the 

 lacrimal bay, the largest area is uncovered, and where, in 

 mammals other than Primates, a special protection is 

 afforded by the membrana nictitans. Irritation of this 

 sort from atmospheric influences, when excessive, gives 

 rise to the diseased conditions known as "pingueculse" and 



