CHAPTER XVII. 



THE SENSE OF HEARING 



1. THROUGH the sense o hearing, we obtain a know- 

 ledge of ; the peculiar vibrations of sonorous bodies, which 

 constitute sounds^. The organ of hearing is very complica- 

 cated, like that of sight, and the precise office of the differ- 

 ent parts is not yet fully known ; it is, however generally 

 divided into the outer, the middle, and the inner part, and 

 the auditory nerve. 



2. The External Ear. /The outer part of the organ of 

 hearing consists of what is called the external ear, and the 

 cavity which leads to the ear-drum ; and is composed chiefly 

 of cartilage, covered with the skin, and supplied with blood- 

 vessels and nerves. Its use is to collect sound, or rather the 

 vibrations of the air, and transmit them through the tube 

 that leads to the ear-drum. For this purpose it is admirably 

 contrived ; its surface being smooth and folded into grooves, 

 which, assisted by a raised border and several concave 

 spaces, conduct whatever sounds fall upon it, with the great- 

 est certainty, directly to the drum of the ear. The external 

 ear is furnished with muscles, and savages are said to have 

 the power of raising or bringing forwards their ears, like a 

 horse, to catch sounds as they come from different direc- 

 tions ; but the habits of civilized life destroy this faculty. 

 The following cut represents the external ear, and the mus- 

 cles by which it is moved. (See Fig. !.) 



3. Various names are assigned to the different portions 

 of the external ear, such as helix to the outer border, scapha, 

 the large boat-like depression, &c. ; but passing these by, as 

 of little importance, we remark that the external ear is con- 

 fined to the class mammalia, nor does it always exist among 

 them, for the mole, the water-shrew, and other diving animals, 



