322 THE BIOLOGY OF BIRDS 



of elastic tissue, and Professor E. W. Carlier (1906) has 

 pointed out that there is relatively more of this in birds than 

 in any other animals. It occurs in the sclerotic, the choroid 

 and various other parts of the eye, and there is also, as hinted 

 at in lizards, a special elastic tendon for the insertion of the 

 ciliary muscle. Now the adaptive significance of this 

 relatively strong development of elastic tissue in the eye is 

 not far to seek. Professor Carlier notes that birds and 

 reptiles are the only Vertebrates with striped muscle fibres 

 within the eyeball, and this is particularly marked in birds 

 which have such powers of rapid accommodation. The 

 powerful and rapid contraction of the muscles would give 

 a shock to the delicate structures within the globe and might 

 interfere with clearness of definition, but there is a means 

 of absorbing this shock, and of converting the sudden pull 

 of the muscle on the choroid into an even, continuous 

 strain. This is effected by the great increase in the amount 

 of elastic tissue within the eye. 



If it be asked why birds should excel other animals 

 in power of accommodation, the answer may be found in 

 their rapid flight from far to near, and in the fact that 

 optical alertness means everything to them. In its rapid 

 plunge on its quarry how quickly the bird of prey must 

 alter its focus ! 



The colour of the bird's eye is determined by various 

 conditions (see Bond, 1919). (i) Thus there may be no 

 pigment on the anterior surface of the iris, and the black 

 of the posterior or uveal epithelium shines through the 

 translucent anterior and results in a black or dark eye — 

 the " bull " eye. (2) There may be no pigment on the 

 anterior surface of the iris, yet the iris may not be trans- 

 lucent owing to its being crowded with colourless granules, 

 which prevent the passage of transmitted light, but when 

 seen by reflected light give the iris a grey-white appearance, 

 the " pearl " eye. (3) The anterior surface of the iris may 

 show a network of branching cells crowded with small 

 yellowish granules, and this results in the " yellow " or 

 " gravel " eye. (4) Many birds have on the anterior 



