VI FORM AND FUNCTION 135 



(i) No species of bird has what can properly be 

 called an external car. The Owl has a Hap of skin, 

 forming a kind of valve, by which it is said that it can 

 close the car at pleasure. Certainly it possesses 

 muscles for this purpose. Often the car valve is larger 

 on one side than the other, the whole skull being at the 

 same time lopsided. During the breeding season, the 

 cock Capercailzie has moments of complete deafness, 

 owing to a fold of skin which becomes swollen with 

 blood and closes the opening of the ear. In other 

 species the flap of skin is very little developed. 



(2) The three small bones which in the human cat- 

 convey the vibrations of sound from the membrane 

 which forms the outer wall of the drum of the ear to 

 the inner membrane that forms a window in the bony 

 labyrinth are represented in birds by one bone, the 

 columella (C). But it is almost certain that this is 

 formed by a fusion of three, corresponding to those 

 which we find in mammals. It was usual, till recently, 

 to see in the quadrate bone, to which the lower jaw of 

 birds and reptiles is hinged, one of the three bones of the 

 mammalian ear. If these three are combined in the 

 columella, where are we to look for the quadrate in 

 man and other mammals ? The best authorities arc 

 of opinion that it is represented only by an insig- 

 nificant ring of bone, called the annulus, which forms 

 a frame for the membrane of the drum of the ear. 



(3) In place of the spiral cochlea birds have a 

 slightly curved bone to which the name of the lagena 

 has been given (Lg). It is similar in reptiles. 



(4) The absence of the organ of Corti in the bird's 

 ear is a remarkable fact. It is true there is a very 



