668 TEXT-BOOK OF PHYSIOLOGY. 



of Mariotte, which consists in placing before the eye two objects having the 

 relation to each other shown in Fig. 328. With the left eye closed and the 

 right eye directed to the cross, both objects may be visible. But by moving 

 the figure away from or toward the eye, there will be found a distance, 

 about 30 cm., when the circle will be invisible. This occurs when the 

 image falls on the optic nerve at its entrance. The experiment of Purkinje 

 as described in the following paragraph demonstrates also the fact that the 

 sensitive portion of the retina is to be found only in the layer of rods and 

 cones. 



It is well known that the blood-vessels of the retina are situated in its 

 innermost layers a short distance behind the optic-nerve fibers. Owing to 

 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. 



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 

 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 Boll that if an animal is kept in 

 darkness an hour or two before death the pigment is massed at the ends of 



