178 THE HUMAN SPECIES 



false impression of obliquity of the eye and a flattening of the 

 nose. 1 



The human eye is further peculiar in permitting a part of 

 the sclerotic to remain uncovered so that the eye appears 

 almond-shaped ; in all other mammals, even in apes, the sclerotic 

 is completely covered by the lids and the eye thus looks round. 



The lachrymal glands for keeping the cornea moist and 

 clean are first met with in reptiles, and attain their highest 

 development in mammals and man. 



Eyelashes are limited to a comparatively small group of 

 the most highly organised mammals, including man and apes, 

 though certain birds (birds of prey, parrots, ostriches, cassowaries) 

 are provided with bristle-feathers round the eyelids which serve 

 to protect the eye in similar manner. 



3. Sense of Smell. Although possibly some degree of 

 olfactory perception may exist in Ccelenterates and Echino- 

 derms, the true sense of smell is first found in worms, where the 

 organs of this sense are probably the ciliated depressions in 

 front of the dorsal portion of the suspensor of the gill in 

 tunicates. 



Pits and depressions with cilia are found among Arthropods 

 (Crustaceans and Insects) ; judging from their habits it must be 

 assumed that some sense of smell is present, though among the 

 lower Molluscs this is still very problematical. 



In Gastropods the olfactory organs were looked for in the 

 tentacles, and there is no doubt that the two hollows surrounded 

 by slight thickening of the integument on the posterior and 

 lower eye margins of Cephalopods discharge this function. 



An actual nose is first met with in Vertebrates, in its simplest 

 form not unlike the structures found in Invertebrates, being 

 merely shallow pits on the head supplied with special nerves. 



The Cephalochordata have a single olfactory depression, 

 all other fishes have paired pits supplied with nerves ; in 

 the Selachii a nasal furrow extending to the angle of the mouth 

 exists. As we proceed higher among the Vertebrates the nose 

 gradually approximates to the human form. Amphibians pos- 

 sess nostrils communicating internally with the respiratory 

 tract with rudimentary fossae in the cartilaginous covering of 



1 Wiedersheim, loc. cit., pp. 160-62. 



