CONTRIBUTIONS FROM PAWLOW'S LABORATORY 371 



auditory stimulus suppresses the action of the established audi- 

 tory conditioned reflex, but the visual reflex is free from any trace 

 of inhibition. The differentiated auditory tone is now repeated 

 several times in succession, in which case not only the auditory 

 but the visual reflex likewise shows unmistakable signs of inhibi- 

 tion. The inhibition starting in the auditory centre radiated 

 beyond its limits and penetrated into other centres in the cortex. 



Here is another very remarkable example. A series of five 

 scratchers are arranged along the hind leg of the dog which 

 irritate the skin mechanically in certain places. The four upper 

 instruments are always used while the animal is being fed, and 

 the irritation of those four spots is a conditioned stimulus of the 

 salivary gland. The fifth scratcher is ineffective owing to the 

 circumstance that its action on the skin has never been accom- 

 panied by an unconditioned stimulus. But if it is employed 

 once, the four others above it lose their influence for a certain 

 length of time and it is possible to observe how rapidly the inhibi- 

 tion spreads to neighboring points in the same centre of the large 

 hemispheres. The more, however, the interval between the action 

 of the fifth and of the other four scratchers is prolonged, the more 

 are the latter freed from the influence of the inhibition wave which 

 gradually recedes to its point of origin. Repeating the same 

 experiment several times, the period of recession of the inhibition 

 wave becomes continually shorter until an inhibitory action is 

 hardly perceptible. 



Corresponding to the very complex relationship between the 

 ever-changing environment and the organism, adapting itself by 

 means of conditioned reflexes, several forms of inhibition have 

 been recognized and described. Sleep is one form of inhibition 

 splitting as it were the existence of the organism into an active 

 and passive phase and either diminishing or completely oblitera- 

 ting all conditioned reflexes. Another important form of inhibi- 

 tion is the external inhibitory process which results from the 

 competition between stimuli affecting the cortex. Every new 

 factor comes in collision with others already acting in the brain 

 and either draws to itself the available supply of energy or is 

 crowded out by the more intense stimuli. In general terms it 

 means that a strongly irritated point in the central nervous 

 system lowers the affectibility of adjacent regions. There is 

 still a third form of inhibition characterized as internal inhibition. 



