VISUAL SENSATIONS 651 



showing that the stimulation of the retina by the outer ring is increased under 

 the influence of contrast. 



On this theory successive contrast phenomena are analogous 

 to certain phenomena we have already studied in other tissues. If 

 extensive breaking down of the visual stuff has been occurring, 

 when the stimulus is removed there will be a swing-back of the 

 condition of the protoplasm of the nerve- endings in the opposite 

 direction, and the catabolic will be replaced by anabolic changes ; 

 just as, on breaking a constant current that has been flowing through 

 a nerve, the condition of raised irritability at the cathode gives place 

 to a condition in which the irritability is depressed below the normal. 



FIG. 296. 



The improving effect on the heart of stimulation of the vagus is 

 also analogous to a successive contrast effect. During stimulation 

 of the vagus the breaking down of the contractile substance is stopped 

 or checked, so that building up or anabolism can go on without 

 interruption. When the excitation of the vagus ceases there is an 

 extra store of contractile material in the muscle-cells. This causes 

 the beat to be more vigorous, and we may say that the increased 

 anabolism has been followed by a period of increased catabolism, 

 just as strong stimulation of a part of the retina with green (anabolism) 

 gives rise to a red after-image (catabolism). 



It seems probable that, as McDougall has pointed out, the examples 

 of simultaneous contrast depend on inhibitory processes analogous 

 to those which we have studied in the spinal cord in dealing with 

 reciprocal innervation and the conditions for the isolation of any 

 effective reflex. Just as the flexor reflex, due to nocuous stimulation 

 of the paw, inhibits the extensor or stepping reflex, by blocking 



