394 



HITCHCOCK'S ANATOMY 



or an ear upon each side of the body, and no internal connecr 

 tion exists between them, so that one may be injured or de- 

 stroyed while the other performs its office. In fact, the inter- 

 nal ear is protected by the hardest bone (the temporal) in the 

 body, and is inclosed in a bony sac to which the only opening 

 of any considerable size is that of the auditory canal. 



FUNCTIONS OF THE EAR. 



720. Functions of the Pinna. — The object of the pinna is 

 to convey sound to the auditory canal. This it does by con- 

 duction and convection ; that is, it acts as a funnel to collect 

 the vibrations of air and transmit them to the auditory canal, 

 and also conducts sounds by means of its own substance in 

 the same manner as a stick of timber conveys the sound made 

 by the scratching of a pin to its opposite extremity with great 

 distinctness. Hence the use of the external ear is to collect 

 sounds from as large a surface as possible, and concentrate 

 them at the small end of a funnel, so as to increase somewhat 

 their intensity. The probable design of the ridges and fur- 

 rows upon the pinna is to give a greater exposed surface to it 

 as well as to receive vibrations more accurately which come 

 in various directions. 



721. Use of the Auditory Canal. — The use of the audi- 

 tory canal is to give greater intensity to the sounds collected 

 by the external ear. This can be illustrated by placing a 

 loug tube close to the ear, while another person speaks at the 

 other extremity. The voice will be greatly increased in in- 

 tensity, because every vibration is transmitted to the ear, and 

 none are dissipated upon the air or surrounding substances. 

 The auditory canal performs the same office, although in a less 

 degree than in the above experiment. This canal also exists 



Is there any communication between the two ears ? 720. What is the object of the 

 pinna? How does it convey sound to the internal ear? Why is the pinna of .«uch an 

 uneven surface ? 721. Of what service is the auditory canal ? How may its use b« illus- 

 trated ? 



