ORGANS OF TKHCEPTION. 



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- 

 mt. 



Although the organs of hearing, in Insects, have not been 



itisfactorily demonstrated, their existence may reasohably 



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, is 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 



