EXTERNAL AND MIDDLE PORTIONS OF THE EAR 763 



CHAPTER LXIII 

 THE EXTERNAL AND MIDDLE PORTIONS OF THE EAR 



The Pinna and Auditory Meatus. — The organ of hearing may be 

 divided into three parts, namely, into the external ear, including the pinna 

 or auricle and auditory meatus, the middle ear, or tympanum, and the 

 internal ear, or labyrinth. The first two are accessory structures and 

 merely serve to direct the undulations in air to the receptor, formed by 

 the organ of Corti of the cochlea. The pinna or auricle is the funnel- 

 shaped expanse of the auditory meatus, consisting essentially of yellow 

 elastic tissue covered with skin. 

 The cap-shaped depression in its 

 center is known as the concha. 



The external ear is especially 

 adapted to collect the sound waves 

 and to reflect them through the audi- 

 tory meatus upon the tympanic 

 membrane. It may be taken for 

 granted, however, that it is not a 

 particularly important part, because 

 many animals lack the pinna entirely 

 without any apparent impairment in 

 the acuity of their hearing, and a 

 person whose pinna has been cut off, 

 can hear almost as well as previously. 

 In many animals, the pinna is beset ^^^' 3 8 1 . — Diagra^imatic Repbe- 



.,1 I 1-1 1 xi SENTATION OF THE DIFFERENT PaBTS OF 



With muscles which are under the the Ear. 



control of the will and are employed l, Pinna; 2, external auditory meatus; 



to change its shape and position and 3, ear drum ; 4, middle ear containing 



to turn it in thp Hirppfinn nf thp the ossicles; 5, Eustachian tube; 6, vesti- 



XO turn It m tne airection OI tne ^^^^^ ^^ ^^e internal ear; 7, auditory 



sound waves. In many instances, nerve; dividing into two branches, one 

 the ears are moved in divergent ^^ ^^^^^ innervates the cochlea and the 

 J- ^„4.- ^ 1-1 , , n • other, the semi-circular canals; 8, paro- 



directions which must naturally give tid gland, 

 a different reflection on the two sides 



and hence, also impart a different quality to the sound as heard by 

 the two ears. This faculty is especially developed in horses and ro- 

 dents. Aquatic animals are in possession of a valve-like mechanism for 

 closing the auditory meatus and many terrestrial animals are capable 

 of enlarging the concha. In man these muscles are evidently of very 

 little importance, because they are retrogressive and cannot, therefore, 

 play a significant part in ascertaining the direction from which the 

 sound is received. The latter faculty seems to originate in the con- 

 jugate deviation of the eyes toward the side from v/hich the sound 

 waves have been projected. 



