OUR FACE FROM FISH TO MAN 



As to the external accessories of vision, man 

 retains a vestige of the nictitating membrane or 

 third eyehd of lower vertebrates in his semilunar 

 fold at the inner corner of the eyelid; but he has 

 advanced far beyond the shark in possessing an 

 elaborate lacrymal or lubricating apparatus, con- 



p2ica. semh^arii 

 ' \ taruncuiCL 



Fig. 101. Tear-dbaining Canals of the Eye (after Keith). 



sisting of tear-producing glands, with two collect- 

 ing canals above and below the caruncula. These 

 two canals converge toward and drain into the 

 lacrymal sac, which is lodged in a pocket of the 

 lacrymal bone on the inner wall of the orbit; 

 the lacrymal sac is continued downward through 

 the naso-lacrymal duct into the nasal chamber. 

 Man also has fleshy, movable eyelids, which are pro- 

 vided with eyelashes and Meibomian glands. 



Many similar details could be cited in which 



the human eye is superior to that of the shark ; but 



194 



