SENSES, INSTINCTS, AND INTELLIGENCE 323 



surface of the iris a network of cells with dark brown or 

 black pigment, resulting in a black eye very different from 

 the " bull " eye (i). (5) Matters become more complicated 

 when there are two kinds of pigment on the anterior surface 

 of the iris. There are some remarkable special cases. 

 Thus in Lawes' Bird of Paradise a brilliant colour effect 

 is produced by a combination of three factors : (a) a trans- 

 lucent iris which allows the uveal pigment to shine through, 

 (b) the absence of anterior yellow pigment-cells in the inner 

 zone of the iris, and {c) a fibrillation of the connective-tissue 

 in that area which acts as a diffusion grating and causes 

 Hght reflected at a certain angle to appear blue. 



In the percipient layer or retina which lines the interior 

 of the back chamber of the eye, there is in our eye a small 

 area of acutest vision, the yellow spot or fovea centralis. 

 The highly complicated retina is here thinned down to the 

 percipient elements known as the rods and cones. In 

 owls, whose eyes look forward, with axes parallel to one 

 another, the central fovea is absent, and its place is taken 

 by a temporal fovea. The same is true of the Oil-bird, 

 Podargus. 



In birds that pick their food from the ground, like 

 curlew and wagtail, and in swimming birds, like ducks and 

 geese, there is a horizontal fovea and a central one as well. 

 In quick-flying birds there is a temporal fovea and a central. 

 In the swift, the swallow, and the tern there are three 

 foveas, so that the finest flight and the most complex retina 

 go together. 



Very characteristic of the bird's eye is the pecten, a 

 vascular pigmented fold, protruding into the vitreous 

 humour of the posterior chamber. It arises from near the 

 " blind spot " where the optic nerve enters the eye and divides 

 into its many branches. It varies greatly in size, sometimes 

 reaching almost to the posterior surface of the lens, some- 

 times being very inconspicuous. It is rudimentary in the 

 Nankeen night-heron {Nycticorax caledoniciis), but it is 

 never absent — not even in the Kiwi (Apteryx), as has been 

 stated. 



