60 



SPECIAL SENSES. 



their object merely to aid the perception of sound with more 

 precision and accuracy. Hence we may conclude that the 

 sense of hearing is dull in animals where the organ is re- 

 duced to its most simple form ; and that animals which have 

 merely a simple membranous sac, without tympanum an! 

 auditory passage, as the fishes, or without semicircular 

 canals, as the crabs, perceive sounds in but a very imper- 

 fect manner. J/ 



3. Of Smell 



105. SMELL is the faculty of perceiving odors, and is 



a highly important 

 sense to many ani- 

 mals. Like sight 

 and hearing, smell 

 depends upon special 

 nerves, the olfacto- 

 ry, (a,) which are 

 the first pair of cer- 

 p. 21 ebral nerves, and 



which, in the em- 

 a, olfactory nerve; b, optic nerve; c, audi- i j- 



, bivo, are direct pro- 



tory nerve ; a, cerebrum ; e, cerebellum ; 



/, nostril. longations of the 



brain. 



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 tne 

 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. Some fishes 

 have four. They are similar openings, separated by a par- 

 tition upon the middle line of the body. In man and the 



