36 FACULTIES OF BIRDS. 



At the bottom of this passage there is a thin 

 parchment-like skin, stretched obliquely across for the 

 reception and reverberation of the sounds propagated 

 thither from the air: this is the drum, which is 

 covered, as we have seen, in the same way as the tube. 

 The middle layer under the drum is quite transparent. 

 It is remarkable that the whole membrane is braced 

 like the head of a drum ; but, in place of the strings 

 of the drum, the membrane of the ear is stretched and 

 kept tight by the threads of a muscle. So, at least, 

 it is conjectured by Sir Everard Home, from analogy; 

 for it is only in large animals, such as the elephant, 

 that these threads have been discovered. 



Bones of the Ear. 



There is placed behind the drum a chain of small 

 bones or springs, so arranged as to hinge upon one 

 another ; and when one moves all the others follow, 

 in consequence of their connection with appropriate 

 muscles. The first of these bones (malleus), which 

 is shaped like a hammer, is in contact with the 

 middle of the drum, and when a sound strikes upon 

 the drum, it is supposed to cause the muscles attached 

 to the bone to contract and pull it, while this move- 

 ment consequently reacts upon the drum. When 

 the motion has been thus originated, it is plausibly 

 conjectured to be communicated in succession to the 

 other three bones of the chain, the last of which 

 (stapes) has the form of a stirrup with a solid base, 

 and plays like the key of a flute upon a hole or 

 opening (fenestra ovalis) that leads to several chan- 

 nels or passages farther inward. The internal ear 



