416 THE EYE IN EVOLUTION 



may be most simply interpreted partly as a mechanical expedient for 

 buttressing the organ to give it rigidity but mainly as a means of 

 increasing the available diffusing surface. From the optical point of 

 view, there is little doubt that a pecten, occupying the space already 

 taken up by the blind spot corresponding to the optic disc, is a more 

 efficient method of nourishing the retina than the provision of a diffuse 

 vascular system whether it be intra-retinal or supra-retinal. Indeed, 

 the position of the pecten is such as to interfere as little as possible with 

 the function of the retina (Petit, 1735), a point to be remembered when 

 considering any possible optical function. In this respect the eyes of 

 birds are optically superior to those of man. 



The most popular subsidiary functions which have been ascribed to the 

 pecten, four of them metabolic, four of them optical in purpose, may most 

 conveniently be summarized as follows : 



(1) An aid in the mechanism of accommodation (Beauregard, 1875 ; Rabl, 

 1900 ; Franz, 1909 ; Hess, 1910). It was suggested that an increase or decrease 

 in turgidity makes the pecten act as an erectile organ capable of displacing the 

 lens hydraulically. It is true that, in general, the size and complexity of the 

 pecten vary with the accommodative capacity, but the accommodative capacity 

 itself varies with the visual effectivity, that is, with the metabolic level of the 

 retina. Any relationship between the two may therefore be parallel rather than 

 causal and there is no evidence that the organ changes in volume with accom- 

 modative adjustments. 



(2) A stabilizer of the intra-ocular pressure, acting as a large capillary- 

 venous reservoir or as an organ of secretion or excretion to regularize the tension 

 of the eye particularly during changes of altitude during flight (Franz, 1909). 



(3) A means of smoothing out the considerable excursion in the ocular 

 pulse -pressure. 



(4) A means of maintaining a high temperature in the eye particularly at 

 high altitudes in an animal with a metabolic rate as rapid as the bird (Kajikawa, 

 1923). 



(5) To screen the retina from the sun's rays from above (Paul Bert, 1875) 

 or, alternatively, to serve as a dark mirror, relaying images onto the retina, 

 particularly from objects above. Thus it has been said to tone down excessive 

 brightness from an image in the sky or, alternatively, to allow a ground -feeding 

 bird to see a predator overhead (Thomson, 1928). 



(6) To intercept rays reaching the eye simultaneously from in front and 

 above (Beauregard, 1875). It is thus held to suppress binocular vision during 

 mojiocular fixation or, alternatively, to suppress monocular diplopia during 

 binocular vision. 



(7) To aid the visual resolution of moving objects when in flight. Menner 

 (1938) suggested that finger-like shadows were thrown upon the retina when the 

 bird looked at the sun ; a moving object would thus be seen intermittently 

 and therefore more clearly as are the spokes of a rotating wheel when viewed 

 stroboscopically. 



(;j) As an aid to navigation. This extraordinary faculty of birds has already 

 beei: ■■ "ussed.^ We have seen that one of the necessities for orientation, 



1 p. 63. 



