EYE OF THE ENGLISH SPARROW 



425 



the fish is second; man, third; reptiles, fourth, and amphibians 

 (the frog) thinnest of all. 



In the sparrow the cones far surpass the rods in number. 

 Schultze ('67) many years ago demonstrated this to be true for 

 day birds, while in night birds he found that the rods predomi- 

 nated. This has been verified by Krause ('94) and many other 

 more recent investigators. As in man, the rods are lackmg in 

 the fovea of the sparrow. The cones in this region are also 

 more slender and longer than in the periphery, as demonstrated 

 by Cajal ('94). According to Fritsch ('11), the cones in the 

 center of the fovea are no longer and often shorter than in the 

 surrounding area centralis. I have not found this condition to 

 obtain in the sparrow. 



TABLE 4 



Relative number of cells in equal areas of the different layers of the retina of the 



adult sparrow at the regions indicated 



Center of fovea 



Edge of fovea (0.326 mm. from center of 



fovea) 



Area centralis (0.816 mm. from center of 



^ovea) _. . . 



Ora serrata 



ROD AND 

 CONE 

 LATER 



25 

 20 



16 



The cones of the sparrow possess an oil droplet, which is 

 located at the junction of the inner and outer segments. This 

 has been described by other investigators in many other species 

 of birds and can doubtless be said to be common to this class of 

 vertebrates. Krause ('94) finds that this oil droplet differs in 

 color in various species of birds. He describes red, yellow- 

 green, orange, and blue colors. Fritsch clahns that the rod-like 

 elements also possess oil droplets. I have not demonstrated 

 them in the true rods of the sparrow. The oil droplet of the 

 cone is shown in figure 36, 0. The inner segments of the cones 

 (/) are slightly longer and thicker than the outer segments. 

 As a usual thing the greatest thickness is near their external 

 ends. This is joined by a long tapering process to the nucleus 



