FKCIIXEirS L.tW 235 



second if called on to do so. The receptors ure not capable of 

 such a rapid recovery. They all take about seven times as long 

 as nerve to recover. They are, therefore, capable of transnntting 

 as a maximum less than 150 impulses per second. This difference 

 in time constitutes a margin of safety, i.e. the ingoing nerve has 

 time completely to recover before the next impulse arrives for 

 propagation. " In fact,'' says Adrian, " the rapid recovery of the 

 nerve fibre makes it practically an aperiodic conducting system as 

 far as the slower end-organ is concerned.'' The message transmitted 

 to the nerve by a receptor will be carried without distortion to the 

 sensory area concerned. 



4. Stimulus and Sensation. We cannot hope to explain on 

 physical grounds the relationship between the nature of the 

 stimulus and the nature of sensation {e.g. why ether waves of a 

 certain frequency falling on the end-organ for sight should give us 

 the sensation of redness, the same waves received by a heat spot 

 arouse a feeling of warmth, etc.) until we can state in physical 

 terms what consciousness is. We do know, however, that the 

 quality (intensity and striking value) of a sensation (emotional 

 factors being kept constant) bears a quantitative relationship to 

 certain physical attributes of the stimulus. Adrian's technique 

 has opened out a vast field for exploration, because consciousness 

 can, in a sense, be eliminated. If we admit that the electrical 

 response of a nerve is quantitatively and qualitatively a symbol of 

 the nervous impulse, then we have an experimental tool of great 

 importance. We can measure the stimulus in energy units and 

 we can measure the frequency, amplitude and duration of the 

 electrical waves induced in the sensory nerve. 



Each receptor has a certain functional inertia and will not 

 respond to stimulation until the energy of the stimulus has reached 

 a minimal value which is specific for each receptor and for each form 

 of stinudus. This threshold value is lowest, as has been said 

 above, for the form of stimulation specific to that organ. Once 

 this value is gained, the resulting sensation bears a definite relation- 

 ship to the incident stimulus until an upper limiting value has been 

 reached, after w^hich increase of stimulation is of no avail. In fact, 

 fatigue rapidly sets in, and the resulting sensation is submaximal. 



Fechner's Law. This law states that the sensation varies as 

 the natural logarithm of the stimulus. Adrian showed that the 

 amplitude of movement of the mercury in the electrometer, and 

 hence the potential difference developed in the nerve, did not vary 

 whatever the intensity of the stimulus. The factors which do 

 vary are the duration of the discharge and, in some cases, the 

 frequency of the discharge. In the case of " phasic " receptors 



