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). 
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¬ 
panic 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 
Keptiles. It contains air; and communicates with the back 
