322 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



is convex. Biconvex lenses are thick at the center and thin on the periphery. 

 Lenses in the eyes of fishes are spherical; in the eyes of mammals they are 

 usually oval and elastic. The sharper the curvature of a biconvex lens, the 

 shorter its focal distance, as in the nearsighted eye (Fig. 17.18). Light pass- 

 ing through a biconvex lens produces a reverse image (Fig. 17.10). Rays 

 reflected from the lower part of an object meet in the upper part of its image 

 and vice versa, creating a small picture that is upside-down. Likewise, the 

 rays reflected from the right side of the object pass to the left side of the 

 image and vice versa, thus the picture is not only upside-down, but its sides 

 are reversed. We learn the proper position of an object by experience and 

 after that we cannot imagine it otherwise. A cat's eyes show a mouse with 

 feet up, but her brain doubtless shows her a mouse with its feet down. 



Image-Forming Eyes. Rays of light reflected from an object fall upon the 

 sensitive receptors and initiate chemical reactions within them which create 

 impulses in the associated nerve fibers. The impulses pass along the fibers to 

 the brain where they are interpreted (Figs. 16.11, 16.22). The number 

 and direction of the light rays and the nature of the receptors on which they 

 fall determine the character of the image they form. Lenses guide light rays 

 to form an image in the eyes of the great majority of animals. There are 

 three main types of image-forming eyes: (1) the exceptional pinhole eyes 

 of Nautilus, in which the rays are brought into diffused focus through a 

 minute hole in the front of each eye (Fig. 17.11); (2) the compound eyes 



optic nerve Pigmented cells 

 of retina 



Fig. 17.11. The pinhole eye of Nautilus, the paper 



''"'"' sailor, a mollusk related to the octopus, is similar to 



the pinhole camera which is in focus for all distances, 



but only a little light is admitted and the image is dim 



and foggy. 



PINHOLE-CAMERA EYE 

 WITHOUT A LENS 



of insects, spiders, and other arthropods, with a lens set into each one of the 

 multiple tubes so that no rays can reach the lens except from directly in front 

 (Fig. 17.12); and (3) the eyes of vertebrates with a single lens set in the 

 front of the eye where it receives light reflected at various angles from the 

 object (Figs. 17.13 and 17.14). The capacities of image-forming eyes are 

 matched by the habits and abilities of their owners to act appropriately for 

 what they see. A fish hawk flying a 100 feet above a lake not only sees a 

 fish beneath the surface but plunges unerringly after it even disappearing 

 into the water to clutch it. The vision of the fish hawk is significant because 



