174 THE SENSES OF ANIMALS 



The fishes possess only an inner ear, but all the vertebrates 

 higher than fishes have at least a middle ear as well. The 

 middle ear and the outer ear, where present, represent the 

 first gill opening of the primitive fishes which, in the sharks 

 and rays, has already become separated firom the other gill 

 slits and is represented by the spiracle, a small opening firom 

 the mouth to the outside just behind the eye. Two of the 

 three little bones in the middle ear of the mammals repre- 

 sent some of the bones round the jaw-joint in lower verte- 

 brates; in the mammals they have been diverted to a new 

 use. In the vertebrates, lower than mammals, that have a 

 middle ear — the birds, most reptiles and the amphibians — 

 there is only one middle-ear bone ; the two others of mam- 

 mals are still part of the jaw mechanism. This single bone 

 corresponds to the stirrup-bone in the mammals and con- 

 nects the ear-drum to the membrane covering the window 

 in the inner ear. It is a more or less straight rod, but the 

 outer part of it is generally gristly and consequently not 

 quite rigid so that it can bend and avoid damaging the inner 

 ear if excessively loud sounds cause too much vibration. 



In birds the inner ear is not coiled and is much shorter 

 than in mammals — at the most it is slightly curved — but 

 near the junction of the upper and lower tubes there is a 

 further patch of hair cells embedded in a cap of jelly that 

 contains minute limey particles. This is called the "lagena" 

 and it is believed to respond particularly to low notes ; when 

 the particles vibrate they touch the hairs of the hair-cells 

 and thereby start a message along the nerves to the brain. 

 It is probable that the acuity of the lagena is less than that 

 of the more complicated part of the ear, and consequently 

 birds probably distinguish a much greater range of high 

 notes than of low ones. It is possible, too, that there is a gap 

 in their range of hearing by the two methods, so that there 

 is a deaf spot between the upper and lower frequencies. 



Birds can obviously hear their own songs and cries — the 

 intricacy of their songs shows that their auditory acuteness 

 and discrimination must be at least as good as ours and 

 perhaps better. Furthermore, the extraordinary power of 

 mimicry possessed by some birds show that they must be 

 able to hear extremely well since some — the mynah even 



