Chap. XII.] 



EAR OF VERTEBRATA. 



465 



on by a membrane (the tympanic membrane) 



which is analogous to, though, of course, not homo- 

 logous with, the membrane which we have already found 

 in the ears of orthopterous insects. Externally to this 

 middle ear is the outer portion, which forms the 

 external auditory meatus, and is in higher forms 

 aided and protected by a concli, or so-called ex- 

 ternal ear. 



Outer division of the ear, The functions of 



Fig. 196. Section through the Head of a Lepidosteus, Six Days after 



Impregnation. 



au v, Auditory pit ; an n, auditory nerve ; ch, notochord ; by, hypoblast. (After 



Balfour.) 



the outer and middle portions is that of conveying the 

 sonorous vibrations to the sensory region internal to 

 them ; they are but poorly developed in the lower 

 forms for two reasons : first of all, the insinking of the 

 sensory portion which is so marked in the higher forms 

 is, as we may expect, less marked in the lower divi- 

 sions of the Vertebrata, for it is a process which has 

 only gradually been effected ; the second reason is to be 

 found in the fact that the trumpet-shaped arrange- 

 ments of the outer region, which are so useful in 

 bringing to a focus the vibrations of air, are not so 

 necessary when the animal lives in water. Evidence 

 as to the influence of this denser medium is afforded us 

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