Release of Function in the Nervous System. 



205 



correlation points to the central origin of these morbid phenomena ; they are 

 not the direct expression, however extensive, of peripheral stimulation. 



Pain and discomfort are the motive force of most elementary reflexes ; any 

 stimulus capable of exciting disagreeable effects is naturally shunned, and 

 the organism tends to turn towards those which are pleasurable. But such 

 protective reactions permit of little choice and they must be controlled on the 

 physiological level in favour of more discriminative responses ; for without 

 such re-adjustment functional development would be impossible. Moreover, 

 in order to acquire some new facility, it may be necessary to inhibit even 

 normal sensory responses, which, from their disagreeable nature, are intended 

 to compel us to avoid the conditions that evoke them. 



For instance, all healthy persons can be made giddy, provided the 

 rotatory movement is sufficiently violent, especially if the head is held 

 in some unusual position. This reaction, even when it culminates in 

 vomiting, is not a pathological condition ; it is the normal response to an 

 acute disturbance of equilibrium. A similar vertigo can sometimes be 

 produced by visual impressions, without any change in bodily posture. But 

 the ease with which these unpleasant states can be evoked differs within 

 wide limits in individual cases and varies greatly at different times. Some, 

 who in childhood were unable to swing without discomfort and were sick on 

 a train-journey, cease to suff'er in adult life, and, provided the stimulus is 

 not unusually severe, differ little in their reactions from their fellows. 



Even in the fully developed nervous system of man, the acquisition of 

 some new aptitude is associated with increasing control over sensory 

 impressions ; we learn to adapt ourselves to the conditions around us, and 

 cease to be the victims of uncoordinated reactions. This is particularly 

 evident when learning to fly, for many normal men become giddy, and 

 even vomit on exposure to rapid changes of posture in the air; but the 

 majority quickly recover, and may become expert and bold aviators. They 

 have adapted themselves to the new conditions; the stimuli from the 

 peripheral end-organs no longer dominate the field of response. They are 

 controlled or suppressed, not by the will, but by that power of adaptation 

 which is one of the most potent automatic factors in the activity of the 

 central nervous system ([8], p. 13). 



Anything which diminishes this physiological efficiency tends to lessen 

 control over the reaction of the lower centres to sensory stimulation. A 

 gastro-intestinal attack, influenza, an aeroplane accident, domestic worry, or 

 fear, will all act in the same direction. A man, who was giddy when he 

 first went into the air but recovered completely, may fall back to his 

 original mode of reaction, and become unable to carry out any acrobatic 



