SENSE ORGANS. 



I8 3 



COMPARATIVE MORPHOLOGY AND PHYSIOLOGY 

 OF THE EAR. 



In no other organ do we find so regular a simplifica- 

 tion in descending the scale of animals. 



In mammals the structure and function of the ear are 

 almost exactly what we described in man. The only 

 important differences are the greater size and efficiency 

 of the external ear as gatherers 

 of sound waves, and the movable- 

 ness of the ear by the use of ap- 

 propriate muscles by which ani- 

 mals perceive direction better 

 than we. These muscles exist 

 even in man, but in a rudimentary 

 and therefore useless condition. 

 The hearing of most mammals is 

 keener than that of man, as they 

 rely much on this sense for their 

 safety. 



Birds. — The first important 

 simplification is found in birds. 

 In the exterior ear the conch is 

 entirely wanting and the meatus 

 is very shallow, so that the mem- 

 brane of the drum is very near the surface of the head. 

 In the mid-ear the chain of bones is reduced substan- 

 tially to one, the columella (which represents the stapes 

 and probably the malleus), and the tympanic cavity is 

 broadly connected with the throat instead of by a slen- 

 der Eustachian tube. In the interior ear we find the 

 cochlea much shorter and uncoiled (Fig. 116). 



Reptiles. — In reptiles the exterior ear is gone; the 

 membrane of the drum is at the surface, covered with 

 skin and often with muscle. The mid-ear is very similar 



Fig. 116. — Interior ear of a 

 bird, showing; cochlea (c) 

 uncoiled. The semicircu- 

 lar canals in three rec- 

 tangular planes are also 

 shown. (From Parker.) 



