REFLEX ACTION 503 



presence of food becomes known increase in number and complexity with the 

 differentiation of receptors. Thus, sometimes one, sometimes another phenomenon 

 of the outer world becomes a sign of food and the impossibility of other than 

 temporary connections is obvious. It is a case of the telephone exchange again, 

 an illustration used also by Pavlov (1910, p. 9). 



How are these temporary combinations made ? How is the conditioned reflex 

 formed ? It is this : if a new, indifferent, external stimulus is many times present 

 along with one which has already a definite response, the subsequent presentation 

 of the new stimulus alone causes the reflex to be given. The reflex arc has now 

 taken into connection with itself an additional afferent neurone, but not for an 

 indefinite time and unconditionally, as we shall see presently. At the risk of 

 some degree of repetition, it seems advisable to give an illustration. A dog, when 

 given food, secretes saliva, as is well known. Suppose that every time the food is 

 given, a particular bell is rung. After a number of repetitions of the combina- 

 tion of bell and food, the sound of the bell alone is found to cause secretion of 

 saliva. A conditioned reflex to the sound of the bell has been formed. This 

 is a very simple case, but the investigation of the various influences to which it 

 is subject leads to a great deal of valuable information. 



The work of Pavlov and his collaborators has, in fact, been hitherto concerned 

 with such a comparatively simple case. The salivary glands have many advan- 

 tages for the purpose. They can work alone, not being parts of a complicated 

 system. The observation of the effect can be made quantitative, by recording the 

 number of drops of saliva secreted. The operation necessary to make a salivary 

 fistula is very simple and does not interfere with the normal state of the animal. 



When food is taken into the mouth, stimulation of the various receptors in 

 the mucous membrane brings about reflex secretion. This is the primitive, 

 unconditioned reflex, present even without the higher parts of the brain. But it 

 can be modified, as every one knows. The sight, or even thought, of food may 

 excite secretion ; this is " psychical " secretion and it requires the highest parts of 

 the brain. Fear may prevent the secretion, so that dry food cannot be swallowed. 

 Now it is just this aspect that can be made into conditioned reflexes. Any 

 phenomenon of the outer world, for which the animal in question possesses 

 appropriate receptors, can be brought into temporary association with salivary 

 secretion, so that it becomes an exciter of secretion, if only it has been frequently 

 presented at the same time with the unconditioned reflex stimulus, food in the 

 mouth. 



The study is concerned with the different kinds of stimuli, their mutual effect 

 on one another, and so on. Since a number may be present at one time, there is 

 a great variety of possibilities of inhibition, of which Pavlov distinguishes two 

 kinds, external and internal. All kinds of external phenomena may give rise to 

 external inhibition. During the course of the formation of a conditioned reflex, 

 especially in the early stages, a very slight outside disturbance may prevent its 

 proper production. Thus, Dr Anrep informs me that he was engaged in the 

 production of a conditioned reflex to a particular metronome beat. It was in the 

 winter time and, just as he presented the food, the laboratory servant began to 

 scrape away the snow at the entrance of the building. The effect of the intended 

 stimulus was at once done away with ; the dog's attention was diverted, as the 

 psychologist would put it, and the experiment spoiled for a time. As regards 

 internal inhibition, it is found that a conditioned reflex, say salivary secietion on 

 the sound of a bell, if repeated several times without the subsequent presentation 

 of food, loses its effect, its proper consummation not being arrived at. This is 

 merely temporary internal inhibition, since the reflex returns of itself after a rest. 



The " analysers " are what have been called sense organs, or mechanisms of 

 sensation, whose function it is to separate and distinguish the complicated 

 phenomena of the outer world. Many of the facts already worked out by physi- 

 ologists belong to Pavlov's category of conditioned reflexes. When, for instance, 

 a certain combination of stimuli, arising from the retina and from the eye muscles, 

 has several times been found to coincide with the touch stimuli of an object of a 

 given sizej the combination becomes the conditioned stimulus of the actual size of 



