CONTRIBUTIONS FROM PAWLOW'S LABORATORY 363 



system, now widely known as the method of conditioned reflexes. 

 In this short time a mass of material has accumulated which 

 could easily fill a volume and the discoveries made by the num- 

 erous workers in that field warrant a thorough revision of our 

 views and teachings regarding the higher functions. 



It is well known that ordinary spinal reflexes, such as sneezing, 

 coughing, etc., are responses to an external irritation and are 

 characterized by the constancy and certainty of their action. 

 The stimulation starting from a sense organ, or nerve ending, 

 travels to the lower region of the central nervous system, where- 

 upon a reaction is set up by a centrifugal impulse conveyed over 

 the motor nerve. These reflexes are essential to the existence 

 of the organism, they are permanent, inherent and, hence, 

 unconditioned. We know of them only their beginning and end, 

 the action and reaction, but we have no knowledge of the manner 

 in which one is transformed into the other, because we invariably 

 find these reflexes in an already formed condition. 



This is quite different in the case of conditioned reflexes. As 

 the name implies, their existence depends upon certain conditions 

 and they appear or disappear under definite circumstances. 

 They arise continually in the nervous system and enter 

 into various temporary associations with other reflexes. In 

 other words, they are exceedingly mobile, uncertain, and tempor- 

 ary in nature. A conditioned reflex is a reaction to an indifferent 

 stimulus, occasioned merely by a repeated coincidence of the 

 latter with a physiologically active stimulus. Its nature is best 

 understood from the manner in which it is ordinarily formed. 

 If a definite sound is produced while the mouth cavity is irritated 

 with acid, and this combination is repeated several times in 

 succession, it will sooner or later happen that the sound will 

 call forth the same reaction which was produced by the acid. 

 The sound has thus assumed a vicarious function and besides 

 affecting the ear, occasions a definite flow of saliva. It has 

 become a stimulus of salivary secretion, and a new salivary reflex 

 is established which, however, is a conditioned reflex, as it is 

 neither permanent nor invariable in its occurrence. 



Conditioned reflexes can be formed also with activities other 

 than the salivary secretion, but this has been used almost ex- 

 clusively by Pawlow and his associates as an indicator of nervous 

 processes. The temporary nature of the conditioned reflexes 



