82 SENSE OF HEARING. 



OF THE SENSE OF HEARING. 



32. Hearing is the sense which enables us to perceive sounds. 



33. Sounds are produced by very rapid oscillatory movements, 

 which are manifested in sonorous bodies, and which are called 

 vibrations. 



34. Sonorous vibrations are communicated, from the bodies in 

 which they are produced, to the surrounding air, and are thus pro- 

 pagated, little by little, or nearer and nearer, like the undulation 

 produced on the surface of smooth water by casting a stone into it. 



35 That sounds may act upon our senses, the oscillatory 

 motion must reach the bottom of the apparatus of hearing, that 

 it may agitate the extremity of the nerve, destined to transmit the 

 sensation which it produces. 



36. The apparatus of hearing is called the ear ; it is double and 

 is symmetrical!} placed on each side of the head. Each of these 

 apparatuses is lodged in the interior of one of the bones of the 

 cranium, called the temporal bone. That part of the temporal 

 Done which contains it, is extremely hard, and for this reason has 

 received the name of [te.tronx honp. 



37. The apparatus of hearing is very complicated in its struc- 

 ture; it may be divided into three principal parts which anato- 

 mists have called the external ear, tke middle ear or carity of the 

 tympanum, and the internal ear, or labyrinth, (Fig. 28.) 



28. The external ear is composed of the parilion of the ear, 

 and the auditory canal, (meatus auditorius externus.) 



39. The external ear, or pavilion of the ear, (Fiu. 28, />.) is 

 a very elastic cartilaginous plate which surrounds the entrance 

 to the auditory apparatus, and presents, in many animals, the 

 form of a trumpet, which serves to direct sounds towards the in- 

 terior of the ear. In man, th^ pavilion of the ear presents many 

 ridges and furrows, or anfractuosities, arising from the folds of 

 fhe cartilaginous plate which forms it. 



40. The auricular canal, or external auditory canal, meatus 



32. What is the sense of hearing ? 



33. How are sounds produced ? What are vibrations? 



34. How are sonorous vibrations propagated ? 



di>. What condition is necessary to produce sensation from sounds? 



36. What is the name of the apparatus of hearing ? Is it double or sin. 

 gle 7 Where is it situated? What is that part of the temporal bone called 

 which contains ihe apparatus of heating? 



37. How is the ear divided ? 



38. What are the divisions of the external ear? 



39. Describe the external ear? 



40. What is the situation of the external auditory canal? What is tha 

 membrana tympani? 



