Comparative Anatomy of the Eye. 445 



that of its relative, the civet, living in an open country, remains round. 

 ( Cuvier. ) The pupil of the sheep's eye is elliptical, but is elongated 

 sidewise instead of vertically, whereby it is enabled to look far to right 

 and left without raising its head, an advantage in grazing. The pupil 

 of the eye acts automatically, contracting as the light increases. Heat 

 on the other hand expands it. These effects can be produced on eyes 

 for a considerable time after they are detached from the living body. 



It has been shown that birds are related more directly to the reptiles 

 than to the mammals. Nevertheless the essential features of the eye are 

 still the same. Many birds whose habits require and cultivate long 

 range of vision and acute discernment of color, have developed details 

 that give them superiority over the mammals in these respects. 



FIG. 177. Eye of Nocturnal Bird of Prey. 

 Co. Cornea. Me Ossifications of the 



L.~ Lens. Sclerotic Coat. 



Tft.-Retina. (7 ^f. -Ciliary Muscle. 



P. -Pecten. 

 CM No. Optic Nerve. (After Wiedersheim.) 



Lizzards, crocodiles, birds, and many fishes 

 have a common property in a peculiar feature 

 called the pecten or marsupium. Huxley de- 

 scribes this as a peculiar vascular membrane 

 covered with pigment like the choroid, which 

 projects from near the entrance of the optic 

 No nerve on the outer side of the globe of the eye 

 FIE. 177. ( ^ e s ide awa J from the nose ) into the vitre- 



ous humor and usually becomes connected with the capsule of the lens. 

 The number of the cones in the eyes of most birds is greater than in 

 mammals, the rule being that in birds the cones are three times as 

 numerous as the rods. They generally have a yellow spot, and in the 

 case of the Falcon and some others there are two yellow spots. This 

 enables the Falcon to concentrate his vision on two objects with equal 

 intensity at one time. Color can be distinguished further than form. 

 Many birds, and reptiles too, have on the ends of their cones minute 

 globules of oil. These vary in color and are light and dark green, 

 yellow, orange, brown, red (carmine), light blue and white. 1 It is sup- 

 posed these globules act like so many special lenses to concentrate the 

 action of their respective colors upon the cones, thus intensifying their 

 sensation. Each one represents to the bird one prime color, and 

 it is able to discern not only this but all the shades that the mixtures of 

 his prime colors will make, when mingled in various proportions and 

 intensities. 



The owl with its nocturnal proclivities manifestly must make small 

 use of a color sense. He possesses but very few cones while he has an 

 extraordinary number oi rods. But his cones are provided with the 



1 Henry T. Finck. 



