336 THE SPECIAL SENSES. 



response of the rod elements. It has been shown that provision 

 exists in the retina for the constant regeneration of this red pigment. 

 It will be remembered that the external segments of the rods im- 

 pinge upon the heavily pigmented epithelial cells that lie between 

 the rods and the choroid coat. From experiments upon frogs' eyes 

 it appears that a portion of the retina detached from the pigment 

 cells and bleached by the action of light is not able to regenerate its 

 visual purple until again laid back upon the choroid coat. This 

 regenerating influence of the black pigmented cells may be con- 

 nected with another interesting relation that they exhibit. Under 

 normal conditions delicate processes extend from these cells and 

 penetrate between the rods and cones. When the eye is exposed 

 to light the black pigment migrates along these processes as far even 

 as the external limiting membrane, and it is possible that this ar- 

 rangement may be useful in obviating diffuse radiation of light 

 from one rod to another. When the eye is kept in the dark, however, 

 the pigment moves outwardly and collects around the external 

 segments, where the process of regeneration of the visual purple is 

 taking place. Further evidence that the visual purple is connected 

 with the irritability of the rods toward light stimulation is shown 

 by the fact that when it is exposed to the different rays of the spec- 

 trum the absorption of light is greatest in that part of the spec- 

 trum (green) which appears the brightest in vision when carried out 

 under such conditions as may be supposed to involve the activity 

 chiefly of the rods (see below for these conditions). It is, however, 

 perfectly obvious that visual purple is not essential to vision. The 

 fact that it is absent from the fovea centralis is alone sufficient 

 proof of this statement. Moreover, it seems to be absent entirely 

 in the eyes of some animals; for instance, the pigeon, hen, some 

 reptiles, and some bats. The most attractive view of the function 

 of the visual purple is that it serves to increase the delicacy of re- 

 sponse or irritability of the rods in dim lights, — a view that is ex- 

 plained in more detail in the paragraph below, dealing with the sup- 

 posed difference in function between the rods and cones. 



The Extent of the Visual Field — Perimetry. — By the visual field 

 of each eye is meant the entire extent of the external world which 

 when the eye is fixed forms an image upon or is projected upon the 

 retina of that eye. From what has been said previously regarding 

 the dioptrics of the eye it is obvious that the visual field is inverted 

 upon the retina, and that, therefore, objects in the upper visual field 

 fall upon the lower half of the retina, and objects in the right half 

 • of the visual field fall upon the left half of the retina. Assuming 

 that the retina is sensitive to light up to the ora serrata, it is evi- 

 dent that if the eye were protruded sufficiently from its orbit its 

 projected visual field when represented upon a flat surface would 

 have the form of a circle, the center of which would correspond to 



