336 



- THE SENSES 



centers. For adequate knowledge as to the detailed structure of the eye, 

 one of the most complicated parts of the wondrous animal body, the 

 reader is referred to text-books of anatomy and histology. Study of 

 such an organic mechanism always much more than repays the time and 

 labor which may be put on it, and in this case is entirely indispensable 

 for any proper understanding of the visual function. Here we are con- 

 cerned only with the way in which its most essential parts enable us to 



FIG. 189 



Diagram of the retina: a, rod- and cone-layer; b, external limiting membrane; c, externaT 

 granular layer; d, external reticular layer; e, internal granular layer; f, internal reticular layer; 

 g, ganglion-cell layer; h, nerve-fiber layer; i, internal limiting membrane. The pigment-layer is 

 not represented. (Bates.) 



see the world's infinite variety of form and color, to somehow perceive 

 objects by waves of light sent into the periphery of the nervous system. 



The eye, then, is essentially a photographic optical instrument for 

 impressing on the extended peripheral termination of the second cranial 

 nerve an image of whatever is placed before it. The nerve then conveys 

 the nature of this image to the brain, and vision results. In order that 

 this image on the retina may be always clear, the eye has lenses and means 



