ORGANS OF PERCEPTION. 208 
concha. In Birds, the tube is short, and there is no ex- 
pansion of the skin to constitute an external ear. In Rep- 
tiles, there is no tube, nor any external opening, (unless in 
the crocodile where the skin forms a kind of lip). The skin 
passes directly over the tympanum, exhibiting no change 
in its direction, and becoming only a little more transpa- 
rent. 
Although the organs of hearing, in Insects, have not been 
satisfactorily demonstrated, their existence may reasonably 
be inferred from the circumstance, that many species are 
capable of producing sounds, and others of acting under 
their influence. 
From the preceding review of the organs of hearing, it 
appears, that in some animals the action excited in the air 
or water by vibrations of sonorous bodies, is communicated 
directly to the auditory nerve, by the medium of the com- 
mon integuments. In other cases, there appears a compli- 
cated apparatus, to collect the vibrations, and to transmit 
them by means of cavities and tubes, variously arranged, 
to the auditory pulp. These differences in the form and 
structure of the parts, must occasion corresponding modi- 
fications in the impressions produced; but with regard to 
the nature of these, we are still in a great measure igno- 
rant. 
2. Knowledge obtained by the sense of Hearing.—The 
information communicated to the mind by the organs of 
Hearing, 1s far from being so important and diversified as 
that which is derived from the sense of touch or sight. 
Sound is produced by the motion of the parts of a par- 
ticular body, or by the friction of one body against ano- 
ther. In both cases, a tremulous motion is communicated 
to the surrounding medium, which extends in all directions 
like the waves produced in the water by a stone falling into 
