60 



SPECIAL SEA'SES. 



their object merely to aid the perception of sound with mora 

 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 and 

 auditory passage, as the fishes, or without semicircular 

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

 fect manner. 



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- 

 ebral nerves, and 

 which, in the em- 

 biyo, are direct pro- 

 longations of the 

 brain. 



106. The organ of smell is the Nose. Throughout tho 

 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. Some fishes 

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

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



Fig. 21. 



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

 tory nerve ; d, cerebrum ; e, cerebellum ; 

 /, nostril. 



