^'''' '^^ 1 I'EWis, Some Considerations on Sight in Birds. 219 



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 l:)lood-vessels ramify over the surface of the retina, 

 and here, also, light is preventeJ from falling on and being regis- 

 tered 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 ])ecten is a pigmented, vascular structure 

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

 tlie papilla of the optic nerve (Plate XXXIII., fig. 2). The size varies 

 considerably in different species, extending in some almost to the 

 posterior surface of the lens, while in others it is small and in- 

 conspicuous. It is absent in one bird — namely, the Apteryx — 

 and is practically absent in the Nankeen Night-Heron {Nycticorax 

 caledonicus). The function of the pecten has always been a 

 matter of controversy. There seem to be no special habits or 

 conditions in birds })ossessing 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 objects it 

 has been found that there is, with the passage backwards of the 

 posterior surface of the cornea, the transference of fluid from the 

 anterior chamber. This is shown by injecting methylene blue 

 into the anterior chamber and stimulating the ner\-es of accom- 

 modation, then noting the course of the fluid. 



Admitting, then, that there is a transference of fluid from one 

 chamber to another to maintain an unvarying intraocular pressure, 

 some governor must be present to effect this quick interchange, 

 and it is believed that the pecten acts in this way. In support of 

 this theory, it can be shown that in high-flying birds, birds of rapid 

 flight, birds of prey, where the eyes have to be accommodated 

 to extremely rapid alteration of focus, the pecten is well developed. 

 It is, on the other hand, comparatively small in nocturnal birds. 

 Against this theory, it may be stated that reptiles, or some 

 reptiles, possess a pecten, and in these animals the above con- 

 ditions hardly exist. The important point is, that the presence 

 of this large pecten creates a large blind area in the eye, and, as 

 it is heavily pigmented, all light falling on it is naturally absorbed. 

 It explains to some extent the constant shifting of the head when 

 a bird is on the watch, as the visual field is considerably limited, 

 the portion obstructed being towards the upper outer field of 

 vision. Before leaving the retina, it should be mentioned that 

 the presence of oil globules in this layer has been known for a long 

 time. These globules are coloured red and yellow, and are found 



