MAMMALIA. 321 



the power of vision is probably limited to distinguishing 

 light from darkness. 



Searing. Sound is conveyed to the mind by the vibration 

 of air, and this vibration is communicated to a membrane 

 within the ear, called the " tympanum ;" this is in connection 



d \ 



FIG. 86. SECTION OF THE HUMAN EAK. 



a, external opening; b, tympanum; c, small bones; d, Eustachian tube ;. 

 e, cochlea;/, semicircular canals. 



(by means of three minute bones) with the expanded audi- 

 tory nerve, which ramifies within the "vestibule," which 

 is filled with a fluid, so that the vibrations of the air are 

 thus communicated to the nerve of hearing (fig. 86) . 



Smell. The interior of the nose is lined by a fine moist 

 membrane, on which the olfactory nerve is distributed. The 

 cause of odour is quite unknown ; though, as most volatile 

 substances are odorous, it is supposed to be some volatile 

 matter which comes in contact with the olfactory nerve, or 

 nerve of smelling. The quantity given off in some instances 

 must be exceedingly small, for one grain of musk placed on a 

 piece of glass will give out scent for months, and at the end 

 of that time weigh still one grain. Some animals, as Dogs 

 and Birds of prey, have this sense so acute that it serves 

 them instead of almost every other ; the Vulture will scent 

 carrion miles away, and the Dog will track his master by the 

 power of scent. 



Taste. The sense of taste resides chiefly in the tongue, 



Y 



