Chap, xii.] EYES OF BIRDS. 455 



In Birds, even more than in Reptiles, we see the 

 influence of the terrestrial mode of life, or rather of 

 the different refractive powers of air and water, in 

 the more convex form of the cornea. It is clear that 

 the rarer the atmosphere the greater is the necessity 

 for a convex apparatus to collect the rays of light, 

 and we find an arrangement in the eyes of flying 

 birds by which this convexity may be attained. By 

 the contraction of the muscles at the sides of and 

 behind the eye, the fluids in its two chambers, and 

 thus the cornea, are pressed forwards ; in some, 

 pressure on the optic nerve is prevented, thanks to 

 the possession of bony plates in the sclerotic. The 

 crystalline lens is flattened, except in Apteryx and the 

 owls that fly by twilight ; the ciliary muscle, which is 

 of such importance in the accommodation of the eye 

 (see Power's " Human Physiology,"), consists in birds, 

 as in reptiles, of striated muscular tissue, whereas in 

 mammals the muscle is of unstriated tissue; owing 

 to the difference in the property of these muscles, the 

 eye of a swif i ly-moving bird is more rapidly brought 

 into focus than is that of the more slowly-moving 

 mammal. It is not, however, unnecessary, perhaps, 

 to point out that this possession of striated tissue in 

 the ciliary muscle of the eye is not to be looked upon 

 as a direct adaptation to the habits of a bird, inas- 

 much as it is possessed also by the more lethargic 

 reptile; all we can say of it is that it is a very 

 useful heritage. Projecting into the hinder chamber 

 of the bird's eye is a folded membrane richly provided 

 with blood-vessels ; this pecten, which is found also 

 in the eye of reptiles, has possibly a nutrient function, 

 but nothing is certainly known as to the office which 

 it fills. Compared with the size of their body, the 

 eyes of birds are large, and the anterior chamber is 

 remarkable for having its longitudinal axis as long as 

 or longer than that of the hinder chamber. Bony 



