VISUAL SENSATIONS 583 



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. 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 



FIG. 298. 



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 innerva- 

 tion 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 the synapses of those elements on the sensory 

 path which would pour their impulses into the final co.mmon path of the 

 extensors, so stimulation of any portion of the retina will tend to inhibit 

 processes of a similar kind occurring in adjacent parts of the retina or their 

 central connections. 



Stimulation of one part of the intestine causes inhibition of the activity 

 of the intestine below the point of stimulation. If this inhibition occurred 



