ii6 INTRODUCTION TO GENERAL PHYSIOLOGY 



The other arrrangement corresponds to the bellows of the 

 camera, to keep out stray light from acting on the plate. It is 

 represented by the eye-ball itself, which is lined by a layer of cells 

 containing black pigment. This pigment layer is to be found in 

 the very simplest eyes, and is clearly of much importance. 



The retina in the vertebrate is a very complex structure of 

 several layers of different kinds of cells (E., p. 214). But several of 

 these layers properly belong, not to the receptor organ itself, but 

 to the nerve centres. In the cuttle-fish they are in a separate 

 nervous mass, outside the eye. The actual receptive layer is that 

 of the rods and cones. That the cones are the elements concerned 

 with accurate vision is obvious from the facts that this is in direct 

 relation to the number of cones present in a given area, and that 

 the central part of the retina, where the most accurate vision is 

 present, contains cones only. The function of the rods is somewhat 

 obscure, but their nervous connections are very similar to those of 

 the cones, and it seems that they must also be percipient elements 

 of some kind. The rods and cones lie in a solution containing 

 visual purple, and when a bright part of an image is formed at a 

 point on the retina, the photo-chemical change in the sensitive 

 substance causes the cones, and perhaps the rods, with which it is 

 in contact to be affected in such a way as to stimulate the nerve 

 fibres in connection with them. Whether this is by a chemical 

 action or by the resonance of molecules to particular wave lengths 

 is not yet clear, but the phenomena of after-images, to be referred 

 to below, suggest that the former is the case. We saw, in discussing 

 the chlorophyll system, that light energy is absorbed by a system 

 for the reason that a certain molecular group has a vibration rate 

 which is in unison with that of the light which it absorbs. The 

 resonant vibrations may be great enough to result in chemical 

 decomposition. 



That a change is produced in the visual purple such that a 

 certain time is necessary for a return to normal is familiar in the 

 negative after-images, where a part of the retina, on which the 

 image of a bright object has fallen, remains for a time less sensitive, 

 thus causing the appearance of a dark patch in the field of view. 

 The regeneration of the visual purple is of interest, because a 

 similar phenomenon is met with in some of the simpler photo- 

 chemical reactions, such as that of silver chloride. Suppose that 

 we have some of this compound in a sealed tube and allow sun- 

 light to act upon it. It turns purple, chlorine being given off, 

 and metallic silver in a finely divided, colloidal form being left. 

 Now chlorine and silver have a strong affinity for one another, 

 and if the tube be placed in the dark they recombine. But 

 this recombination takes place whether light is acting or not, so 



