414 ORGANS OF THE SENSES. 



membrane comes in direct contact with the external air, 

 although it is placed at one end of a collecting and concen- 

 trating apparatus (composed of the pinna or auricle of the 

 ear and of the external auditory canal). We can best under- 

 stand this by reducing the internal ear to a drop of liquid, 

 and supposing a vibrating membrane (membrane of the 

 fenestra ovalis and the base of the stapes) which vibrates by 

 the intermediation of a solid chain, the chain of the ossicles, 

 the other extremity of which is connected with a collecting 

 organ, the membrana tympani, and the cavity of the concha. 

 As the second membrane (the deepest, the fenestra ovalis) is 



Fig. 109. Diagram of the auditory apparatus in man.* 



much smaller than the first (membrane of the tympanum or 

 drum), the slightest vibration of the latter will set in action 

 the former. We will now proceed to study the office of these 

 parts, taking them in an opposite direction or from without 

 inwards, which is the direction which the waves of sound 

 follow. 



A. External ear. 



The pinna of the ear or concha is an organ which has little 

 sensibility of its own, and whose general and tactile sensibility 

 is somewhat dull ; the ornaments with which, even among civil- 



* From right to left are seen the external ear, the auditory passage, the tym- 

 panum with the chain of small bones, the Eustachian tube, and the labyrinth. 

 (Dalton, "Human Physiology.") 



