SIGHT 139 



example the seals, have no naso-lachryma] ducts — they spend 

 so much of their lives in the water that they hardly need 

 them. Consequently when seals come out to sleep on the 

 land their tears overflow on to their faces and when their 

 coats are dry the wet patches round their eyes make them 

 appear to be weeping copiously. 



The snakes differ from most of the land vertebrates in 

 having no movable eyelids ; each eye is covered by a single 

 transparent scale. It is difficult to suggest a plausible reason 

 for this arrangement, for many of the lizards which have a 

 very similar way of life, have normal eyelids. Yet there must 

 be some advantage to these reptiles in dispensing with eye- 

 lids because some of the lizards have done so too, either 

 partly or completely. In some lizards the lower eyelid is a 

 transparent scale or "spectacle" so that the animal can see 

 even when the eye is shut ; in yet others the eyelids have 

 grown together so that the spectacle is permanently in place. 

 There is little doubt the snakes have lost their eyelids by a 

 similar evolutionary process. 



We are so used to having only two eyelids that it seems 

 peculiar that some animals should have three, yet a third 

 eyelid is the rule rather than the exception in the land verte- 

 brates. It can easily be seen in birds, and most people must 

 have noticed it in action in their household cat. It is a 

 semi-transparent membrane that folds away at the inner 

 corner of the eye inside the paired lids. It is used for clean- 

 ing the front of the eye globe and works sideways, unlike 

 the outer lids that work up and down. We ourselves carry 

 a degenerate and functionless third eyelid in the little pink 

 speck at the junction of the eyelids beside the nose. It is 

 peculiar that when a mammal shuts its eyes the upper lids 

 move down but when most birds do so the lower lids move 



up. 



In addition to their cleaning function the outer eyelids 

 protect the eye from injury, shut out disturbing visual stimuli 

 during sleep, and allow the light-sensitive structures at the 

 back of the eye to rest when they are not required to be in 

 action. The eye is further protected from injury, and shaded 

 from strong light, by the fringe of eyelashes on the edge of 

 the eyelids. The eyelashes vary in their development but 



