THE NERVOUS SYSTEM AND SPECIAL FUNCTIONS. 299 



the animals that smell by means of a localized organ : first, 

 that the substance be capable of giving off minute portions 

 of itself which are so much lighter than air that they move 

 suspended in it, and are readily carried by currents of air. 

 It is supposed that aquatic animals do not smell as air- 

 inhabiting animals do; though it is possible that they 

 become sensible of odors by some other means, as by 

 solutions in water. The body membrane, underneath 

 which the branches of the olfactory nerve are spread, must 

 have its mucous surface soft and moist. These atoms of 

 the odor-bearing substance, striking this surface and being 

 moistened by its fluids, affect it in such a way as to bring 

 about the sensation of some particular thing, as violets, 

 roses, new-mown hay, fried potatoes, ploughed ground; 

 or to the vulture, carrion, its sense in this respect being 

 keener than the sense of man for the same odor. 



As to the smelling organ of the bird, the nostrils open 

 externally upon the upper mandibles; and the nasal 

 chamber back of each nostril communicates with the 

 mouth, smell being connected with taste, as it is in the 

 human tribe. The olfactory nerve enters each nasal 

 chamber by a single opening ; and its branches are spread 

 under the membrane lining the chamber. 



Hearing. 



Birds rival man in their receptivity to musical har- 

 monies, and in the extent to which they are aroused by 

 musical vibrations of the atmosphere. Quickness and 

 accuracy of hearing are remarkable in the genus of birds 

 to which the mocking bird belongs. He will render 

 chance notes more accurately than can ninety-nine out of 

 a hundred human beings. Yet a bird's ear is structurally 

 very simple. There is usually no external ear, but some 



