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TEXT-BOOK OF PHYSIOLOGY 



this anatomic arrangement, a portion of the light coming through the pupil 

 will be intercepted by the vessels and a shadow projected on the layer of 

 rods and cones. Ordinarily, these shadows are not perceived, for the reason 

 that the shaded parts are more sensitive, so that the small amount of light 

 passing through the vessels produces as strong an impression on this part 

 as .does the full amount of light on the unshaded parts of the retina, and 

 perhaps because the mind has learned to disregard them. But if light be 

 made to enter the eye obliquely, the position of the shadows will be changed, 

 when at once they become apparent. This can be shown in the following 

 way: If in a darkened room a lighted candle be held several inches to the 

 side and to the front of the eye, and then moved up and down, there will 

 be perceived, apparently in the field of vision, an arborescent figure corres- 

 ponding to the retinal blood-vessels. This is due to the falling of the 

 shadows on unusual portions of the layer of rods and cones. 



FIG. 309. SECTION OF THE RETINA OF A FROG. A. In darkness. B. In light. 

 (Ajter Van Genderen Stort, from T scheming s "Physiologic Optics."') 



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 layers of rods and cones 

 alone remaining, and in the fovea only the cones are present. That this 

 area is the point of 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 irregu- 

 larly diminishes from the macula toward the periphery. This diminution 

 in insensibility 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 unknown. The discovery of 

 the visual purple in the outer segment of the rods gave promise of some 



