RECEPTOR ORGANS 



cones of birds, and in the corresponding structures of the retina of the frog, fish, 

 and cephalopod. Since it is not present in the cones of the human eye, it is absent 

 from the region of sharpest vision, the fovea centralis, a fact which has led some 

 observers to doubt whether it has any real importance. Edridge-Green, however, 

 has brought forward evidence to show that it diffuses into the fovea from the 

 surrounding rods. The rods, them- 

 selves, he regards as being concerned 

 only with the formation of the pig- .5 

 ment and not receptor organs for ? 

 light. As regards this last point, it 

 appears that the sensibility of the 

 various zones of the retina in the 

 recognition of form is directly pro- 

 portional to the number of cones per 

 unit area which they contain. Put in 

 another way, the images of two points 

 are recognised as distinct according to 

 whether they fall on two cones or not, 

 so that they must be further apart to 

 be recognised as two in the peripheral 

 parts of the retina, where the cones 

 are further apart. But the micro- 

 scopical appearance of the cell con- 

 nections of the rods is very similar to 

 that of the cones and does not suggest 

 that of secretory cells only. 



Kuhne showed that the pigment is 

 sensitive to light while in the eye, and 

 that photographs of objects can be 

 made on the 

 fact. These 

 (1878, p. 225). 



The difficulty of obtaining infor- 

 mation as to the chemical nature of 

 visual purple, as it has become the 

 custom to call it, is obvious, on 

 account of the very small quantity to 

 be obtained. Its solubilities are 

 peculiar ; according to Ktihne, it is 

 only dissolved by bile salts with readi- 

 ness. This fact suggests a colloidal 

 suspension ; the lowering of surface 

 tension produced so powerfully by 

 these substances would facilitate a 

 great dispersion of the particles. It 

 does not, in fact, diffuse through 

 parchment paper, so that the bile salts 

 can be removed by dialysis. In addi- 

 tion to dispersion of the pigment, the 

 bile salts appear to disintegrate the 

 rods. The pigment is not attacked 

 by trypsin, so that it is not of protein 



nature. The method used by Kuhne to obtain his purest preparations will be 

 found on p. 454 of his paper with Ewald (1878), and on p. 266 of his article 

 in Hermann's " Handbuch " (1879). 



When light falls on the peripheral parts of the retina in man, it is found that, 

 when diminished so as to be just visible, it is only that part of the spectrum 

 between wave lengths 600 and 440 fj./j. (orange to blue) that is visible at all, and 

 the sensation is one of light only, without colour, whatever the wave length 



retina owing to this 

 are called optograms 



FIG. 164. CURVES CORRELATING THRESH- 

 OLD OF VISUAL SENSATION, ACTION OF 

 LIGHT ON THE VISUAL PURPLE, AND 

 THE ABSORPTION OF LIGHT BY VISUAL 

 PURPLE. 



Ordinates relative values in units of energy required 



to produce effect. 

 Abscissse wave length of light. 



(Victor Henri et Larguier des Bancels, 

 1911, 1, Fig. 4.) 



