652 PHYSIOLOGY 



the synapses of those elements on the sensory path which would 

 pour their impulses into the final common 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 on both sides of the stimulated spot we should have 

 a phenomenon exactly analogous to the process of simultaneous contrast. 

 In the same way the increased briskness of the antagonistic reflex 

 which follows the temporary inhibition accompanying the primary 

 reflex can be compared to the negative after-image resulting on 

 prolonged stimulation of any point in the retina, while alternating 

 after-images, positive and negative, which may succeed a single 

 positive stimulation, have their analogue in the alternating rhythmic 

 movements of flexion and extension of the two legs which may follow 

 single stimulation of one of them. The phenomena of binocular 

 contrast show that the retinas are not alone concerned in the pro- 

 duction of these phenomena, but that we are dealing also with the 

 central connections of the optic nerves, the activities of which must 

 be regulated by the same laws as those determined from our study 

 of the more lowly activities of the spinal cord. 



