374 SERGIUS MORGULIS 



especially if his manner is at all provocative, causes a furious 

 outburst of an aggressive reaction on the part of the dog. The 

 stranger proceeds with the experiment in spite of the dog's 

 excitement, applies the familiar conditioned stimulus, and to 

 the general astonishment, gets an unparalleled response from the 

 salivary gland. If he continues the experiment longer and the 

 dog gradually becomes quiet the conditioned salivary reflex 

 commences to fade and at last the secretion ceases. The least 

 movement on the part of the stranger will at once rouse the 

 dog's fury and immediately thereupon the conditioned salivary 

 reflex will wax strong again and the story will repeat itself. 



This experiment has wide theoretical bearings, revealing 

 clearly the interaction of different centres of activity in the hemis- 

 pheres. An intense irritation of the aggressive centre radiates 

 out from the point of origin and invades the other centres in 

 the brain, causing a general increase of the nervous tonus of the 

 cortex. This involves also the secretory centre. But as the 

 tide of excitement recedes to the aggressive centre the above 

 formulated law of competition of co-existing factors comes into 

 prominence, an external inhibitory process sets in and as a result 

 of that the action of the salivary centre is suppressed. 



Here is still another very striking case. The skin of a dog is 

 irritated by an electric current of such strength as to cause a 

 painful sensation (or a destructive action, in accordance with the 

 objective terminology). Each time this stimulus is applied the 

 mechanism of self-defense is set into a vigorous reaction ; the ani- 

 mal attempts to break loose from the stand, to snatch the instru- 

 ment, and so on, in other words, a strong defensive reflex results. 

 If food is given to the dog at the same time (this must frequently 

 be done through a stomach tube) it sooner or later comes about 

 that the defensive reaction is gradually subdued and at last 

 vanishes altogether, while the electrical irritation becomes a 

 conditioned stimulus of the salivary gland. Two important 

 things must be considered in this connection. First, the blocking 

 of the irritation of one centre (defensive) by the activity of another 

 (nutritive) and, secondly, the diversion of the stimulus into a 

 new channel towards a point in the cortex of more intense func- 

 tion. The nutritive centre 4 which is physiologically very im- 



4 Pawlow is of the opinion that the animal possesses a nutritive centre which 

 resembles in every essential the respiratory centre. It is beyond the limits of this 

 article to discuss this point further. I wish to make, however, this comment upon 



