358 COORDINATION AND SENSATION 



mastered, this being necessary to their practical use in 

 exchanging ideas. From spoken language man has ad- 

 vanced to written language, so that the sight of the written 

 or printed word also arouses in the mind the associated 

 idea. 



THE EAR 



The Ear is the sense organ which enables sound waves 

 to so act upon afferent neurons as to excite impulses in 

 them. The effect upon the mind which these impulses 

 produce is known as the sensation of hearing. In the 

 performance of its function the ear receives and transmits 

 sound waves and also concentrates them upon a suitable 

 exposure of nerve cells. It includes three parts the 

 external ear, the middle ear, and the internal ear. 



External Ear. The external ear consists of the part 

 on the outside of the head called the pinna, or auricle, and 

 the tube leading into the middle ear, called the auditory 

 canal (Fig. 151). The pinna by its peculiar shape aids to 

 some extent the entrance of sound waves into the auditory 

 canal. 1 It consists chiefly of cartilage. The auditory canal 

 is a little more than an inch in length and one fourth of an 

 inch in diameter, and is closed at its inner end by a thin, 

 but important membrane, called 



The Membrana Tympani. This membrane consists of 

 three thin layers. The outer layer is continuous with the 

 lining of the auditory canal ; the inner is a part of the lin- 

 ing of the middle ear ; and the middle is a fine layer of 

 connective tissue. Being thin and delicately poised, the 

 membrana tympani is easily made to vibrate by the sound 



1 It is only the central portion of the pinna that aids the entrance of sound into 

 the auditory canal. If by accident the outer portion of the pinna is removed, there 

 is no impairment of the hearing. 



