FIGURE NO. 22 



THIRD EYELID IN MAN AND BIRD 



In the inner upper corner of man's eye (to the left) 

 there is a vestige of a third eyelid or nictitating mem- 

 brane (A). It is larger in some races than in others, 

 but it is of no use. In most mammals, however, as in 

 rabbit and dog, it is well-developed and is drawn over 

 the front of the eye for cleaning purposes. Its absence 

 in man and monkeys is compensated for by the fre- 

 quent movements of the upper eyelid. Its absence in 

 whales is compensated for by the continual washing 

 of the front of the eye. In birds it is usually an im- 

 portant membrane (a, in figure to the right), used in 

 cleaning the front of the eye. It is drawn down by 

 special muscles and goes back spontaneously when 

 they relax. 



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