196 GENERAL ZOOLOGY 



cones, which are the percipient elements of the eye, it is neces- 

 sary for the light to traverse the various layers of the retina lying 

 between them and the inner surface of the retina. The rods 

 and cones having been stimulated by the light, the nervous 

 impulse set up passes in the reverse direction from the rods and 

 cones to the fibers forming the optic nerve which convey the 

 impulse to the brain. The cones are concerned in both chromatic 

 and achromatic vision, while the rods function only in achromatic 

 vision. Thus colors are perceived by the cones alone. 



Accommodation. — The human eye resembles a photographic 

 camera in many respects. The curved cornea, distended by aque- 

 ous humor, and the lens, provide a means for converging light 

 rays, as in a camera, and the sensitive retina corresponds to the 

 film or plate of the camera. Rays of light entering the eye from 

 a luminous object, or from an object reflecting light, are brought 

 to a focus at a point behind the lens, beyond which an image is 

 formed on the retina. The human eye possesses a range of vision 

 that makes it possible to see objects miles away or close at hand. 

 The power of accommodation for objects at varying distances is 

 due to the fact that the focal length of the lens can be changed. 

 In the eye of some molluscs and in the eye of fishes, accommoda- 

 tion is brought about by altering the distance between the lens 

 and the retina, as in a camera, where the lens is moved away 

 from the film for near objects and in opposite direction for more 

 distant objects. In these cases the focal length of the lens is 

 fixed. In the human eye, the same end is achieved by changing 

 the curvature of the lens rather than by moving the lens as a 

 whole. Thus in viewing near objects the curvature of the lens is 

 greater than when more distant objects are looked at. Increas- 

 ing the curvature of the lens shortens the length of the focus, and 

 decreasing the curvature lengthens it. According to the gener- 

 ally accepted explanation, this adjustment is brought about by 

 the contraction of the ciliary muscles. When the eye is relaxed 

 or unaccommodated, the suspensory ligament and the capsule 

 containing the lens are taut, and the front surface of the lens is 

 flattened, thus reducing its curvature, with the result that distant 

 objects are seen without effort. In viewing near objects the lens 

 becomes more convex as a result of the contraction of the ciliary 

 muscles which pulls forward the region of attachment of the 

 suspensory ligament to the eyeball, thus easing the tension on 



