728 HANDBOOK OF PHYSIOLOGY. 



The visual purple cannot however be absolutely essential to the due 

 production of visual sensations, as it is absent from the retinal cones, 

 and from the macula lutea and fovea centralis of the human retina, and 

 does not appear to exist at all in the retinas of some animals, e.g. , bat, 

 dove, and hen, which are, nevertheless, possessed of good vision. 



However the fact remains that light falling upon the retina (a) 

 bleaches the visual purple, and this must be considered as one of its effects. 

 It. has been found that certain pigments, also sensitive to light, are con- 

 tained in the inner segments of the cones. These colored bodies are said 

 to be oil globules of various colors, red, green, and yellow, called chromo- 

 phanes, and are found only in the retinas of animals not mammals. The 

 rhodopsin at any rate appears to be derived in some way from the retinal 

 pigment, since the color is not renewed after bleaching if the retina be 

 detached from its pigment layer. (V) The second change produced by 

 the action of the light upon the retina is the movement of the pigment 

 cells. On the stimulation of light the granules of pigment in the cells 

 which overlie the outer part of the rod and cone layer of the retina 

 become diffused in the parts of the cells between the rods and cones, the 

 melanin or fuscin granules, as they are called, passing down into the pro- 

 cesses of the cells, (c) A movement of the cones and possibly of the rods 

 is also said to occur, as has been already incidentally mentioned ; on the 

 stimulus of light the outer parts of the cones, which in an eye protected 

 from light extend to the pigment layer, are retracted. It is even 

 thought that the contraction is under the control of the nervous system; 

 and finally, according to the careful researches of Dewar and McKen- 

 drick, and of Holmgren, it appears that the stimulus of light is able to 

 produce (d) a variation of the natural electrical currents of the retina. 

 The current is at first increased and then diminished. McKendrick 

 believes that this is the electrical expression of those chemical changes 

 in the retina of which we have already spoken. 



VISUAL PERCEPTIONS AND JUDGMENTS. 



Reversion of the Image. It will be as well to repeat here that 

 the direction given to the rays by their refraction is regulated by that 

 of the central ray, or axis of the cone, toward which the rays are bent. 

 The image of any point of an object is, therefore, as a rule (the exceptions 

 to which need not here be stated), always formed in a line identical with 

 the axis of the cone of light, as in the line of B , or A a (fig. 432), so that 

 the spot where the image of any point will be formed upon the retina 

 may be determined by prolonging the central ray of the cone of light, 

 or that ray which traverses the centre of the pupil. Thus A a is the 

 axis or central ray of the cone of light issuing from A ; B b the central 



