THE SENSE OF SIGHT. 627 



to the falling of the shadows on unusual portions of the layer of rods 

 and cones. 



Excitability of the Retina. The retina is not equally excitable in 

 all parts of its extent. The maximum degree of sensibility is found 

 in the macula lutea, and especially in its central portion, the fovea. 

 In this region the layers of the retina almost entirely disappear, the 

 layer of rods and cones alone remaining, and in the fovea only the 

 latter are present. That this area is the point o] most distinct vision 

 is shown by the observation that when the eye is directed to any given 

 point of light, its image always falls in the fovea. Any pathologic 

 change in the fovea is attended by marked indistinctness of vision. 

 The sensibility of the retina gradually but irregularly diminishes from 

 the macula toward the periphery. This diminution in sensibility 

 holds true for monochromatic as well as white light. 



As stated above, the nature of the molecular processes which take 

 place in the retinal tissue, caused on one hand by the light vibrations, 

 and on the other hand developing nerve impulses, is entirely un- 

 known. The discovery of the visual purple in the outer segment of 

 the rods gave promise of some explanation of the process, especially 

 when it was shown to undergo changes when exposed to the action 

 of light. But as the pigment is wanting in the cones, and especially 

 in the fovea, it cannot be considered essential to distinct vision, 

 although that it plays some important role in the visual process is 

 highly probable. It was observed by Van Genderen Stort, that 

 when an animal is kept in darkness some time before death, the 

 cones are long and filiform; but if the animal has been exposed to 

 light, they are short and swollen. It was discovered by Ball that 

 if an animal is kept in darkness an hour or two before death the 

 pigment is massed at the ends of the rods and cones, but after ex- 

 posure to light it becomes displaced and extends over and between 

 the rods almost to the external limiting membrane. These condi- 

 tions are represented in Fig. 302. 



The Eye a Living Camera. In its construction, in the arrange- 

 ment of its various parts, and in their mode of action the eye may be 

 compared to a camera obscura. Though the comparison may not be 

 absolutely exact, yet in a general way it is true that there are many 

 striking points of similarity between them; e. g., the sclera and chorioid 

 may be compared to the walls of the camera; the combined refracting 

 media to the single lens, the action of which results in the focusing 

 of the light rays; the retina to the sensitive plate receiving the image 

 formed at the focal point; the iris to the diaphragm for the regulation 

 of the amount of light to be admitted, and for the partial exclusion 

 of those marginal rays which give rise to spheric aberration; the ciliary 

 muscle to the adjusting screw, by means of which the image is brought 

 to a focus on the sensitive plate, notwithstanding the varying distances 



