360 COORDINATION AND SENSATION 



ing the membrane. The Eustachian tube admits air 

 freely to the middle ear, providing in this way for an equal- 

 ity of atmospheric pressure on the two sides of the drum 

 membrane. The bridge of bones and the air in the 

 middle ear receive vibrations from the membrana tympani 

 and communicate them to the membrane of the internal ear. 



Purposes of the Middle Ear. The middle ear serves two important 

 purposes. In the first place, it makes it possible for sound waves to 

 set the membrana tympani in vibration. This membrane could not 

 be made to vibrate by the more delicate of the sound waves if it were 

 stretched over a bone, or over some of the softer tissues, or over a liquid. 

 Its vibration is made possible by the presence of air on both sides, 

 and this condition is supplied, on the inner side, by the middle ear. 

 The Eustachian tube, by providing for an equality of pressure on the 

 two sides of the membrane, also aids in this purpose. 



In the second place, the middle ear provides a means for concentrat- 

 ing the force of the sound waves as they pass from the membrana 

 tympani to the internal ear. This concentration is effected in the fol- 

 lowing manner : 



1. The bridge of bones, being pivoted at one point to the walls of 

 the middle ear, forms a lever in which the malleus is the long arm, and 

 the incus and stapes the short arm, their ratio being about that of three 

 to two. This causes the incus to move through a shorter distance, but 

 with greater force than the end of the malleus. 



2. The area of the membrana tympani is about twenty times as great 

 as the membrane of the internal ear which is acted upon by the stapes. 

 The force from the larger surface is, therefore, concentrated by the 

 bridge of bones upon the smaller surface. By the combination of these 

 two devices, the waves striking upon the membrane of the internal ear 

 are rendered some thirty times more effective than are the same waves 

 entering the auditory canal. 



The Internal Ear, or labyrinth, occupies a series of ir- 

 regular channels in the petrous process of the temporal 

 bone. 1 It is very complicated in structure, and at the 

 same time is very small. Its greatest length is not more 



1 The inner projection of the temporal bone is known as the petrous process. 



