616 



SCIENCE. 



[N. S. Vol. XXIV. No. 620. 



the giving of certain kinds of food, or by 

 forcing certain substances into the dog's 

 mouth. These artificial conditioned re- 

 flexes, the product of our training, showed 

 exactly the same properties as the natural 

 conditioned reflexes previously described. 

 As regards their obliteration and restora- 

 tion, they followed essentially the same laws 

 as the natural conditioned reflexes. 



Up to the present time the stimuli with 

 which we had to do were comparatively few 

 in number, but were constant in action. 

 Now, however, in another more complicated 

 portion of the nervous system we encounter 

 a new phenomenon — namely, the condi- 

 tioned reflex. On the one hand, the nervous 

 apparatus is responsive in the highest de- 

 gree—that is, it is susceptible to the most 

 varied external stimuli, but, on the other 

 hand, these stimuli are not constant in their 

 operation and are not uniformly associated 

 with a definite physiological effect. The 

 introduction of the idea of conditioned re- 

 flexes into physiology seems to me to be 

 justified because it corresponds to the facts 

 that have been adduced, since it represents 

 a direct inference from them. It is in 

 agreement with the general mechanical 

 hypotheses of natural science. It is com- 

 pletely covered by the ideas of paths and 

 inhibition, ideas which have been sufficiently 

 worked up in the physiological material of 

 the present day. Finally, in these condi- 

 tioned stimuli, looked at from the point 

 of view of general biology, there is nothing 

 but a very complete mechanism of accom- 

 modation or, which amounts to the same 

 thing, a very delicate apparatus for main- 

 taining the natural equilibrium. There 

 are reasons for considering the process of 

 the conditioned reflex to be an elementary 

 process— namely, a process which really 

 consists in the coincidence of any one of the 

 innumerable vague external stimuli with 

 a stimulated condition of any point in a 

 certain portion of the central nervous sys- 



tem. In this way for the time being a 

 path is made by which the stimulus may 

 reach the given point. 



Although there are differences in the 

 time required for the establishing of the 

 conditioned reflexes, some proportionality 

 may be perceived. From our experiments 

 it is very evident that the intensity of the 

 stimulation is of essential importance. In 

 contradistinction to this we must state with 

 regard to acoustic impressions that very 

 powerful stimuli, such as the violent ring- 

 ing of a bell, were not, in comparison with 

 weaker stimuli, quick to produce condi- 

 tioned increase of function in the salivary 

 glands. It must be supposed that power- 

 ful acoustic stimuli produce in the body 

 some other important reaction which hin- 

 ders the development of the salivary re- 

 action. 



What is it that the nervous system of 

 the dog recognizes as individual phenomena 

 of external origin ? or, in other words, what 

 are the elements of a stimulus'? If the 

 application of cold to a definite area of the 

 skin acts as a conditioned stimulus of the 

 salivary glands, the application of cold to 

 another portion of the skin causes secretion 

 of saliva on the very first occasion. This 

 shows that the stimulus of cold generalizes 

 itself over a considerable portion of the 

 skin, or perhaps even over the whole of it. 



Stimulation by musical sounds or by 

 noise in general is remarkably convenient 

 for determining the discriminating or 

 analytical faculty of the nervous system 

 of the dog. In this respect the precision 

 of our reaction goes a great way. If a 

 certain note of an instrument is employed 

 as a conditioned stimulus, it often happens 

 that not only all the notes adjoining it, but 

 even those differing from it by a quarter 

 of a tone, fail to produce any effect. 

 Musical timtre is recognized with similar 

 or even much greater precision. 



We have hitherto spoken of the analytical 



