202 PHILOSOPHY OF ZOOLOGY. 



These bones are susceptible of motion, and have muscles 

 attached to them, by which they are supposed able to 

 stretch the drum, to compress the fluids in the labyrinth, 

 and communicate the vibrations of the drum to the pulp of 

 the canals and cochlea. 



In Birds, the tympanum bears a close resemblance to 

 that of quadrupeds. Its cellular openings, however, are 

 more numerous, and the cells themselves of greater dimen- 

 sions. The fenestra ovalis and rotunda, are placed, the 

 former above the latter, and separated by an osseous bar. 

 The Eustachian tube is osseous throughout its whole 

 length. In the barrel there is but one bone connecting the 

 drum with the labyrinth, branched where united with the 

 drum, proceeding by slender stalks through the cavity of 

 the barrel, and forming a plate which closes the fenestra 

 ovalis. 



In Reptiles, the tympanum exhibits very remarkable 

 differences. In some, it can scarcely be said to exist, while 

 in others, it is membranaceous, exhibiting the Eustachian 

 tube, and one or two bones in the barrel. The membrane, 

 or drum, is wanting in some cases, its place being supplied 

 by the common skin, while in others, it exhibits its usual 

 characters of dryness, transparency, and elasticity. 



In Fishes, particularly those with free branchiae, there is 

 no tympanum. In those with fixed branchiae, the rudi- 

 ment of a tympanum may be observed in the form of a 

 minute tube leading from the labyrinth to the skin. 



Between the drum and the surrounding clement in Qua- 

 drupeds, the External Ear is situated. It consists of a 

 tube continued from the drum to the surface of the body. 

 It varies greatly in the length, direction, and texture of its 

 walls. When arrived at the surface, this meatus audito- 

 rium ejcternus, as it is called,- either terminates in a simple 

 hole, or it is expanded into a cartilaginous arch, termed the 



