148 THE SENSES OF ANIMALS 



directed more forwards and less to the side than usual. They 

 thus sacrifice the power of all-round sight in exchange for 

 stereoscopic vision which helps in judging distance and speed. 

 They also have two foveas in each eye; in addition to the 

 central fovea there is a temporal one which is used in looking 

 ahead in stereoscopic vision. The sharpness of sight in birds 

 is also increased by the absence of a network of blood vessels 

 such as covers the retina of mammals. The blood vessels are 



IRIS 



PUPIL 

 LENS 



Fig. 4. The eye of a bird cut in half to show the internal structure, especially 

 the pecten. 



carried in a thin membrane, the pecten, that projects as a 

 fold into the posterior chamber from the blind spot, and the 

 cells of the retina get their nourishment by diffusion from 

 these blood vessels through the vitreous humour. Although 

 the pecten is very transparent it is an obstruction, and one 

 would think that it must impair the vision. In fact it helps 

 vision because it casts a slight irregular shadow on the retina ; 

 this, by varying the mean illumination of the retina, has the 

 effect of increasing contrast when a small object crosses the 

 field of view against a uniform background such as the sky. 

 It is significant that the pecten is especially prominent in the 

 eyes of hawks and eagles. On the other hand, in owls, the 

 nocturnal birds of prey, it is small, but in these birds acuity 

 is low for other reasons though sensitivity is great. In owls 

 the retina consists predominantly of rods, there is no fovea, 

 the aperture of the iris is comparatively large and there is 

 little power of accommodation. The result of these peculiari- 

 ties is that an owl has to put up with a rather blurred picture 

 — but that it can get a picture when we should get none at 



