THE SENSE OF SIGHT. 



625 



regard is the angle gamma, while the angle between the optic 

 axis and the line of vision is the angle beta. In emmetropia the 

 angle alpha is about 5. In hypermetropia it is greater, amount- 

 ing to 7 or 8, giving to the eye an appearance of divergence. 

 In myopia it is much smaller 2 or in extreme cases may be 

 abolished, the line of vision corresponding with the optic axis 

 or even passing beyond it. The angle gamma is of value in de- 

 termining the relative power and action of the extra-ocular 

 muscles. 



Functions of the Retina. Of all the layers of the retina, the 

 rods and cones appear to be the most essential to vision. It is only 

 this layer that is capable of-receiving the light stimulus and of trans- 

 forming it into some specific form of energy, which in turn arouses 



Temporal tfuf-e 



FIG. 300. DIAGRAM SHOWING THE CORNEAL Axis D D, THE OPTIC Axis O A, THE 

 VISUAL Axis V L, AND THE LINE OF FIXATION V C; ALSO THE THREE ANGLES, 



a, (3, y. 



in the fibers of the optic nerve the characteristic nerve impulses, 

 ray of light entering the eye passes entirely through the various 

 layers of the retina, and is arrested only upon reaching the pigmentary 

 epithelium in which the rods and cones are embedded. As to the 

 manner in which the objective stimuli light and color, so called 

 are transformed into nerve impulses, but little is known. It is prob- 

 able that the ether vibrations are transformed into heat, which 

 excites the rods and cones. These, acting as highly specialized end 

 organs of the optic nerve, start the impulses on their way to the brain, 

 where the seeing process takes place. As to the relative function of 

 the rods and cones, it has been suggested, from the study of the facts 

 of comparative anatomy, that the rods are impressed only by differ- 

 ences in the intensity of light, while the cones, in addition, are im- 

 pressed by qualitative differences in color. _The nerve-fibers them- 

 40 



