218 The Vertebrate Organ Systems 



eye. Here the cones are most concentrated and daylight vision is most 

 acute. In general, our vision relies to a great extent upon focusing the 

 images perceived on this fovea centralis. 



As can be seen, the perception of objects is due to the interaction 

 of these rods and cones, with the rods perceiving the blacks and whites 

 and the cones the colors. At night when colors are gradually lost, the 

 cones cease to function and vision is mainly a matter of rod function. 

 Thus night vision can he improved by looking side wise at an object 

 rather than directly; that is, the image should be formed to one side 

 of the fovea centralis rather than directly upon it. 



At the point where the optic nerve leaves the retina there are no 

 rods or cones. This is called the blind spot and no visual sensations 

 can be initiated there. This spot can be demonstrated by means of a 

 simple experiment. With Fig. 68 focus your right eye on the + while 

 your left eye is closed. Hold the diagram about six inches away from 

 your eyes and slowly move it away. At the point where the dot dis- 

 appears from view the light rays from the dot are focused on your blind 

 spot. 



+ 



Fig. 68. — Demonstration of the blind spot. 



The actual focusing of the image upon the retina is accomplished 

 by the crystalHne lens. By action of the ciliary muscles, the shape of 

 the lens is altered. Thus accommodation is dependent upon the action 

 of these muscles and the elasticity of the lens, and varies in different 

 individuals of varying ages. For near vision, the lens bulges, while for 

 distant vision it flattens. The rays of light from an object are bent 

 by the lens so that they converge on the retina. As the rays enter 

 the eye, they cross one another and an inverted image is formed on 

 the retina. Through learning, this image is interpreted by the brain 

 in such a way that objects are seen in their correct relationships. 



The quantity of light entering the eye is regulated by the size of 

 the pupil which in turn is determined by the pupillary reflex. In bright 

 light the pupil is small, in dim light it is large. This pupillary reflex 

 also works closely with the accommodation mechanism. In accommo- 

 dating for near objects, the pupil is small and allows less light to 

 enter; for far objects, a maximum amount of light is admitted. 



