580 VERTEBRATE LIFE AND ORGANIZATION 



within the ciliary body. This brings the point of origin of the zonule 

 fibers a bit closer to the lens and releases the tension of these fibers. 

 The front of the elastic lens bulges out slightly, and its refractive 

 powers are increased accordingly. When the ciliary muscles are re- 

 laxed, intraocular pressure pushes the wall of the eyeball outward, 

 increases the tension of the zonule fibers, and the lens is flattened 

 a bit. The lens becomes less elastic with age, and our ability to 

 focus on near objects decreases. 



The refractive parts of the eye form a sharp image of an object 

 on the retina only in an eyeball of appropriate length. If the eyeball 

 is shorter than normal, as it is in far-sighted people, the image of an 

 object in theory falls behind the retina. Accommodation is necessary 

 to bring the image into focus, and the power of accommodation 

 may not be great enough to locus on a near object. This can be 

 corrected by placing a convex lens in front of the eye (Fig. 29.4 B 

 and C). Near-sighted people have eyeballs that are longer than normal 

 and the image falls short of the retina. This can be corrected by a con- 

 cave lens (Fig. 29.4 D and E). 



Light that strikes the rods and cones activates them and they in turn 

 initiate nerve impulses. Recent studies have given us an indication of 

 some of the steps in this process. Each rod contains a light-sensitive pig- 

 ment known as rhodopsin (visual purple). Exposure to light causes this 

 to split into its components, a protein (opsin) and retinene (visual yel- 

 low), and the rod is activated in the process. Recovery involves the re- 

 synthesis of visual purple from its components. Since retinene is an 

 aldehyde of vitamin A, a person suffering from a severe vitamin A de- 

 ficiency does not have as much visual purple as a normal person and 

 cannot see as well in dim light. A cycle of breakdown and reconstitution 

 of rhodopsin goes on continually if the eyes are exposed to any light. 

 The cycle, however, is influenced by the amount of light, for visual 

 purple breaks down faster in bright light, and is reconstituted faster in 

 the dark. To see well in dim light one should stay in a dark room for a 

 while so that a maximum amount of visual purple is reconstituted. 



The cones contain a light-sensitive pigment known as visual violet 

 (iodopsin), which is composed of retinene and a protein different from 

 that in visual purple. The action and biochemistry of visual violet are 

 less well understood. 



Eyes o^ Other Vertebrates. The eyes of all vertebrates are essen- 

 tially alike, but those of primitive vertebrates differ from mammalian 

 eyes in several important respects, for the problems associated with sight 

 beneath water are not identical with those in the air. For one thing, 

 the water itself cleans and moistens the eye, and fishes have not evolved 

 movable eyelids or tear glands. Secondly, the refractive index of water 

 is nearly the same as that of the cornea, so the cornea of a fish's eye 

 does not bend light rays. Most refraction is accomplished by the lens, 

 which is nearly spherical and hence has a greater refractive power than 

 the oval lens of tetrapods. It is interesting in this connection that the 

 lens of a frog's eye flattens a bit during metamorphosis, when a change 

 in environment occurs. Finally, the method of accommodation differs. 



