ORGANS OF PERCEPTION. 905 
Thus, we can in general distinguish between the sound of 
wood or metal, water or air, and hollow or solid bodies. 
In judging, however, of any one of these qualities or 
conditions of bodies, we call to our assistance all the infor- 
mation which is communicated by the different characters 
of the sonorous vibrations, so that the conclusion at which 
we arrive, is frequently the result of a very complex, though 
rapidly executed mental operation. 
To animals in general, the sense of hearing gives warn- 
ing of the approach or retreat of their foes. In some cases, 
it is hkewise the medium of communication between the 
individuals of the same species, in the expressions of their 
feelings of joy or grief, love or hatred. The human ear, 
judges of all the qualities of sounds. Some of the qua- 
drupeds and birds can, however, perceive sounds which 
are inaudible to us, and perhaps can recognise more readily 
than we are able to do, some of the modifications of a par- 
ticular quality. But our knowledge of the peculiar func- 
tions of the different parts of the ear, is still too limited to 
enable us to speculate, with any prospect of success, even 
were our information concerning the habits and feelings of 
the lower animals more extensive and precise. 
In the systematical arrangement of animals, the charac- 
ters furnished by the organs of hearing, are scarcely ever 
employed, unless in the exhibition of generic or specific dis- 
tinctions. Even for such purposes, the attention is exclu- 
sively directed to the external ear, and to the form and po- 
sition of the entrance ; the internal ear being concealed from 
view, difficult to dissect, and furnishing characters which 
can scarcely be expressed by words. 
V. SENSE OF SMELL. 
In those animals which possess organs of Smell, suffi- 
ciently developed to be obvious to the eye, the Nose, or en- 
