THE ORGANS OF SENSATION i 79 



the air-passages. In reptiles, the nasal tubes run side by side 

 separated by a thin vertical lamella forming a nasal septum, and 

 shut off from the mouth by a palatal plate, while true carti- 

 laginous, though rudimentary, fossae form the nostrils of birds. 



All Vertebrates have three bony fossae, of which the upper 

 two are formed from the ethmoid bone, while the lowest fossa 

 is developed independently : great differences exist among 

 mammals as regards the development of the nasal fossae. The 

 highest differentiation is met with in carnivora and ruminants, 

 the lowest in marsupials, whales, web-footed animals, apes and 

 man. 



FIG. 89. Outer wall 01 right half of the nose. (From Thome's Zoologie.) a, b, 

 c, the nasal fossae, covered with mucous membrane; h, hard palate; i, soft 

 palate; f, upper part of pharynx; k, orifice of Eustachian tube ; p, plexus of 

 oliactory nerve ; 9 and b', branches of fifth nerve ; o, upper lip. 



In the human embryo, apart from a number of accessory 

 sinuses, two or three other main fossae are seen, which in later 

 stages disappear practically completely, 1 so there would seem to 

 be some grounds for Klaatsch's suggestion that the sense of smell 

 in man is less highly developed than in other animals because 

 theieyes have gradually encroached upon the nasal cavities. 



The olfactory nerve is distributed in all mammals over the 



1 Wiedersheim, loc. cit., p. 149. 

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