234 OUTPOSTS OF THE INTELLIGENCE SERVICE 



merely whispers, while all other types of change have to shout for 

 attention. 



f* 2. Adaptation. A change in the environment of suitable nature 

 and of sufficient intensity to be perceived by the central area 

 concerned, ceases after a time to produce any effect. Change, 

 ceaseless change alone, is capable of being conveyed to conscious- 

 ness. The steady state is unproductive of alterations in the 

 nervous system. We cease to be aware of the steady pressure of 

 our clothes, of the regular ticking of the clock, of the peculiar 

 odour of our laboratories and so on. Three structures are concerned 

 in this : (a) the receptor, (b) the sensory nerve, and (c) the central 

 sensory area. The adaptation may take place in any or in all of 

 these. From Adrian's work we know that sensory nerves adapt 

 themselves very rapidly and practically all at the same rate. This 

 information is obtainable from experiments where the electrical 

 changes that accompany the nervous impulse are made to record 

 their own fluctuations (p. 225). When a sensory nerve fibre is 

 stimulated electrically, only one wave of negativity passes along 

 the nerve. Continued similar stinuilation produces no further 

 effect. Adrian dissected out an end-organ with sensory nerve 

 intact, e.g. eye of eel and optic nerve, hair and cutaneous nerve, 

 etc. He found that when an adequate stimulus was applied to the 

 receptor, the electrometer, oscillograph, loud-speaker, or other 

 electrical device, showed a burst of electrical activity lasting only 

 a fraction of a second, even though the stimulation was continued. 

 That is, there is a rate of adaptation for nerve plus receptor which is 

 different from that of nerve alone. All the end-organs adapt 

 themselves more slowly than nerves alone. Some, like those of 

 touch and tickle, are fairly rapid, others, like pressure and muscle 

 joint sense, are very sIoav. The former are termed phasic, and are 

 concerned with stimuli demanding rapid action, e.g. touch-jump, 

 tickle-scratch, etc. The latter are jjostural, conveying information 

 about the position occupied by our limbs — resistance to move- 

 ment, weights held, etc., not necessitating any very rapid response, 

 but calling for the exercise of further mental processes, e.g. 

 discrimination. That is, we are given time for the sensation 

 aroused by the stimulation of a postural end -organ to " sink in." 

 About adaptation in the sensory area, nothing can be said in the 

 present state of knowledge. 



3. Frequency of Discharge. In the previous chapter we con- 

 sidered the refractory period of nerve. This period, during which 

 the nerve cannot transmit any propagated impulse, is, for mam- 

 malian sensory nerves, somewhat less than 1/1,000 second. That 

 means that the nerve could conduct at least 1,000 impulses per 



