6o8 SCIENCE PROGRESS 



life and experience. Such cortical reactions, although in their 

 essential outline inherited and instinctive, are yet capable of 

 education and modifiable by training. In so far as they are 

 conscious they are properly matter for psychology rather than 

 physiology. Yet physiology can advantageously study them in 

 regard to their sources in the reactions of afferent nerves, to the 

 interaction of nervous centres which they involve and to their 

 expression by effector organs as muscular and glandular acts. 



A reflex is at its simplest a nervous reaction linking a 

 particular muscular or glandular activity to a particular form 

 of stimulus. Thus in reflex salivation the introduction of food 

 into the mouth is linked to a secreting of saliva, the link itself 

 being an intervening nervous reaction. This nervous reaction 

 involves the brain but not the cortex of the brain. Yet the 

 dog's cortex under ordinary circumstances becomes a factor 

 conditioning the reflex. With it the centripetal channels of the 

 reflex open up side connections and are largely extended 

 thereby. Food when signalled not only by the tongue but by 

 the eye, or nose, or ear becomes through the intermediation of 

 the cortex able to call forth saliva. The cortex is a great nodal 

 area where manifold influences meet and are liable to coalesce 

 or conflict. Elicited through cortical channels the salivary reflex 

 is liable to be disturbed by many conditions and therefore 

 to be variable in reaction. Hence associate reflexes such as 

 these obtainable through the cortex are termed " conditioned " 

 (Pawlow), in contra-distinction from the practically inevitable 

 reflexes obtained through the primal bulbar or spinal channels. 



Not only are these conditioned reflexes less certain but they 

 have plasticity. They can be modified by training and certain 

 forms of them can be acquired by training. They can be acquired 

 by training as so to say side attachments to the aboriginal 

 reflex. Thus a visual stimulus such as a letter of the alphabet 

 can by training be made to call forth a secretion of saliva. The 

 letter, after being presented to the eye on a number of occasions 

 as a regular precedent to the offering of food, becomes by 

 association a visual stimulus able to excite secretion of saliva. 

 A conditioned reflex is thus established. It is not innate but 

 is acquired by the experience of the individual. Conditions of 

 the moment particularly powerfully affect such reflexes. 



Suppose a conditioned salivary reflex has been acquired for 

 a visual stimulus— ^.^. the letter A. Suppose another and 



