36 



SPECIAL SENSES. 



auditory passage, and even the semicircular canals, have for 

 their object merely to cause the perception of sound with 

 more precision and accuracy. Hence we may conclude 

 that hearing is dull in animals where the organ is reduced 

 to its most simple form ; and that animals which have 

 merely a simple membranous sac, without tympanum and 

 auditory passage, as the fishes, or without semi-circular 

 canals, as the crabs, perceive sounds but in a very imperfect 

 manner. 



105. 



a> 



3. Of Smell. 



SMELL is the faculty of perceiving odors. Like 



sight and hearing, 

 smell depends upon 

 special nerves, the 

 olfactory (a), which 

 form the first pair 

 of cerebral nerves, 

 and which, in the 

 embryo, are direct 



Fig. 21. Head of a Crow. prolongations of the 



a, olfactory nerve ; b, optic nerve ; c, auditory i 

 nerve ; d, cerebrum ; e, cerebellum. 



106. The organ of smell, is the NOSE. Throughout the 

 series of vertebrates, it makes a part of the face, and in 

 man, by reason of its prominent form, it becomes one of the 

 dominant traits of his countenance ; in other mammals, the 

 nose loses this prominency by degrees, and the nostrils 

 no longer open downwards, but forwards. In birds, the 

 position of the nostrils is a little different ; they open farther 

 back and higher, at the origin of the beak. 



107. The nostrils are usually two in number. They are 

 similar openings, separated by a partition upon the middle 

 line of the body. In man and the mammals, the outer walls 

 of the nose are composed of cartilage ; but internally, the 



