520 PRINCIPLES OF GENERAL r/fYSfOLOGY 



The Pigment Cells of the retina are also excited to movement by the action of 

 light, and here again the use of the phenomenon remains problematical. 



The " Dermatoptic Function" described by Raphael Dubois (1892), is of interest, 

 as it shows the possibility of light absorbed by pigment acting as stimulus. The 

 siphon of the mollusc, Pholas, is sensitive to light and is retracted when light falls 

 upon it. The response is due to the presence of pigmented cells in the epithelium, 

 prolonged into contractile fibres. According to Dubois, the reception of the light 

 stimulus results in contraction of the fibre, which contraction then, in some way, 

 stimulates nerve fibres going to centres and thus setting off a reflex contraction of 

 the siphon. 



Steinach (1892) states that pigmented muscle cells are to be found in the iris 

 and that these cells can be seen, under the microscope, to contract when light falls 

 upon them. The observations were confirmed by Guth (1901). 



Changes have been described in the Ganglion cells of the retina, but these 

 clearly must be regarded as effects of prolonged stimulation on the cells of nerve 

 centres. 



The Visual Purple. There is every reason to believe that the means by 

 which light stimulates nerve endings is through a photo-chemical reaction. There 

 are an enormous number of chemical reactions which are affected by light, and, 

 of these, one is known in connection with the retina, namely, the changes in 

 the " visual purple." Whether this is the only one we cannot say with certainty, 

 but we shall see presently that its properties are in extraordinary coincidence 

 with certain aspects of vision. As will be shown in Chapter XIX., if a substance 

 is sensitive to rays of a particular wave length, such as would be necessary to 

 account for colour vision, it must absorb these rays. Since they must be in the 

 region of the visible spectrum, the substance must have an absorption band in 

 the region referred to, and, therefore, be itself possessed of colour. Although 

 visual purple is the only such substance detected as yet in the retina, with the 

 exception of certain coloured globules, which are not sensitive to light, described 

 by Kiihne (1878), in the cones of birds, it is conceivable that others may be present 

 in the small amount required, and even in the requisite number, to account 

 for the number of colours to which the eye is sensitive. It is, moreover, not 

 impossibfe. that visual purple, as obtained, may be a mixture of a number of 

 different substances, each with an absorption for a particular group of wave 

 lengths and giving rise to its own particular photo chemical product, to which 

 a definite receptor only is sensitive. More probably, in the formation of the 

 particular photo-chemical product, molecular vibrations of a certain rate are excited, 

 possibly by resonance, with the excitation of receptors by the energy set free. 



Although it had been known for some time that the retina of a frog, removed 

 in the dark, appeared to be of a red or purple colour, when observed in the light, 

 and that the colour disappeared more or less rapidly, the definite association of the 

 pigment in question with visual sensation was not made until Boll's work (1876), 

 followed by the more extensive and detailed work of Kiihne and his fellow- workers 

 (1878). 



The colour of the pigment is not exactly what most people would call purple, 

 it contains much more red. But, having a trace of violet in it, it is best described 

 as a deep pink or rose colour. 



It is bleached by light, but, in the retina, the colours return in the dark. 

 Whether there is new pigment formed or whether the products of the action of 

 light return to their original state in the dark, a very common phenomenon in 

 photo-chemical reactions, is not altogether certain. It appears, however, that 

 under some conditions, solutions of the pigment recover their colour when allowed 

 to stand in the dark after being bleached by light. It has no isolated absorption 

 band in the spectrum, but absorbs light almost equally in all parts, leaving a little 

 red and violet, hence its colour. It is to be expected, then, that it would be 

 responsive to light of all wave lengths, except the extreme red and violet. As 

 indicated above, a series of substances with absorption bands along the course of 

 the spectrum, when mixed together, might give a similar continuous absorption. 



Visual purple is found in the rods only of the mammalian eye, in the so-called 



