CONSIDERATIONS ON SIGHT IN BIRDS — LEWIS. 339 



There is little to be said of the iris in birds apart from the fact 

 that the movement of this curtain or diaphragm is voluntary, the 

 pupil widening or closing at will. Apart from the voluntary 

 action, closing of the pupil or a stopping-down process occurs in 

 the presence of strong light, and is, therefore, reflex in nature, 

 widening of the pupil being noticed in weak light and also for 

 distant vision. 



The retina — the sensitive plate, as it were, of the eye — consists of 

 a layer of fine nerve endings which in most animals conform to 

 two well-marked types — rods and cones. In birds it has been for 

 a long time thought that this layer consisted of rods only, but 

 closer examination shows that cones are present, though very much 

 reduced in number. There is also a belief existent, with perhaps 

 some reason, that the function of the cones is associated with differ- 

 entiation of colors or the formation of visual purple, while rods 

 determine movement, form, and shape. This is the layer which is 

 stimulated by the photo-chemical action of light, the sensitizing 

 substance being found in the external layer of the retina and called, 

 for convenience, visual purple. It is believed that this substance 

 changes under the effect of light, and the chemical changes effected 

 act on and stimulate the nerve endings, giving rise to the particular 

 sensation. In vertebrates this retina is not without its drawbacks. 

 There is a well-marked blind spot where the optic nerve branches 

 out into its numerous endings, this area being particularly large 

 where the pecten is well developed. Further, many blood vessels 

 ramify over the surface of the retina, and here, also, light is pre- 

 vented from falling on and being registered by the sensitive layer. 



It is well known that in man there is a central small area where 

 sight is keenest. This is called the fovea centralis^ and here only 

 rods are present. In birds it is believed that there are two such 

 areas in each eye, one on either side of the pecten. It may be 

 stated here that the pecten is a pigmented, vascular structure lying 

 in the posterior chamber of the eye, protruding forward from the 

 papilla of the optic nerve (pi. 1, fig. 1). The size varies consider- 

 ably in different species, extending in some almost to the posterior 

 surface of the lens, while in others it is small and inconspicuous. It 

 is absent in one bird — namely, the Apteryx — and is practically absent 

 in the Nankeen night heron {Nycticorax caledordeus) . The function 

 of the pecten has always been a matter of controversy. There seem 

 to be no special habits or conditions in birds possessing this structure 

 of equal size and shape, while birds with similar habits show great 

 variations. One theory was that it was protective, guarding the 

 retina from the action of excessive light, in other words, a light filter. 

 Its structure being vascular suggests some functions associated with 

 the tension or nutrition of the eyeball. In accommodation for near 



