EXTERNAL EAR : TYMPANIC CAVITY. 405 



hollows and elevations on the surface of the external ear of 

 Man are not very apparent; but it is probable that they 

 direct the sonorous undulations towards the entrance of the 

 canal which leads to the middle ear. The form of the external 

 ear in many Quadrupeds evidently adapts it to this purpose ; 

 and there are several which have the power of changing its 

 direction by muscular action, in such a manner as to enable it 

 to catch most advantageously the faintest sounds from any 

 quarter. This is especially the case with animals that are 

 naturally timorous, such as the Hare or the Deer ; these have 

 usually very large external ears. But it is among the Bat 

 tribe whose residence in the dark recesses of caverns and 

 excavations makes their eyes of comparatively little use to 

 them, and causes them to depend greatly for guidance in their 

 movements upon the sense of hearing that we find the 

 greatest development of the external ear (fig. 201). 



Fig. 201. -LONG-EARED BAT. 



516. The canal hollowed -out in the temporal bone (d, 

 fig. 204) into which the external ear collects the sonorous 

 vibrations, passes inwards until it is terminated by a mem- 

 brane stretched across it, which is called the membrana tym- 

 panij or membrane of the drum of the ear (g). This forms 

 the outside wall of a cavity (h) which constitutes the middle 

 ear, and which is bounded on the inside by a bony wall that 

 separates it from the internal ear. The cavity of the tym* 

 panum is not one of the essential parts of the organ ; for 

 nothing analogous to it exists either in Fishes or in the lower 

 Eeptiles. It contains air ; and communicates with the back 



