THE BUILDING OF REFLEX ARCS 243 



of this work. His previous work on the digestive glands carried 

 on by delicate operations in which the oesophagus was diverted from 

 the stomach and made to open externally, and in which a portion 

 of the stomach was diverted from the rest and a new " small 

 stomach " was formed, gave him the opportunity of immensely 

 important insight into the factors governing the work of the various 

 glands of the stomach. The work of others showed similar results 

 in the pancreas. I only refer to these because they lead up to the 

 special artificial results with new reflexes which he described in 

 1913. He states that the nervous system besides the primitive 

 function of reproducing innate reflexes, possesses another prime 

 function — namely the formation of new reflexes ; and that the 

 living thing is enabled to respond, by definite and suitable activities 

 to agencies to which it was formerly indifferent. His experiments 

 on the formation of " conditional reflexes," as he calls them rather 

 than " acquired " as opposed to " innate," are grouped around the 

 feeding of the animal and mainly deal with the salivary glands, 

 because they are in direct connection with the external world and 

 their reactions are simply and easily observed. An indifferent 

 stimulus is chosen for the reflexes which it is desired to build up, 

 and this is applied at the same time as food or acid is introduced 

 in the mouth. After a few sittings it is found that this indifferent 

 stimulus alone is now capable of calling forth a secretion of saliva. 

 " The conditional reflex has been formed ; the formerly indifferent 

 stimulus has now found a path to the requisite part of the central 

 nervous system. The reflex-arc has now a different afferent 

 neurone." He gives a good example of this in the result of the 

 application of painful stimuli by a strong electrical current to the 

 skin, systematically accompanying each feeding of the animal. 

 He finds that the strongest electrical stimuli applied to the skin 

 give rise merely to the " feeding reaction," that is, the secretion 

 of saliva, and no indications of any fright or pain appear. " The 

 skin of a dog can be subjected to cutting, pinching or burning, and 

 the only result we shall obtain will be the manifestation of what, 

 judging from our own experience, we should call the symptoms ot 

 the keenest appetite ; the animal follows the experimenter about, 

 licks himself, and saliva flows in abundance." This, it must be 

 remembered, occurs in the absence of the offer or sight of food, at 

 the time in question. He adds : " In this way we have been able 

 to divert the impulses from one path to another according to the 

 conditions, and we cannot avoid the conclusion that the diversion 

 of an impulse from one path to another represents one of the most 

 important functions of the highest parts of the central nervous 



b2 



