240 OUTPOSTS OF THE INTELLIGENCE SERVICE 



light, too hot or too cold an object touching the body. Recent 

 work, however, leads one to the conclusion that although abnor- 

 mally intense stimuli may cause a painful sensation, there are 

 specific receptors for pain. For example, pain spots may be 

 demonstrated in the skin, in much the same way as touch spots, 

 using a needle in place of a soft bristle. Further, pain may be 

 elicited by the stimulation of surfaces known to contain no other 

 receptors, e.g. cornea. The electrical response on stimulation of a 

 pain spot is of similar frequency (5-100 a second) to that produced 

 by touch. It differs in its duration and intensity. A slight pain 

 would be one accompanied by an electrical discharge lasting, say, 

 1 second, while the discharge during acute pain may last about 

 20 seconds. Touch produces an electrical burst lasting about 

 0-2 second. It may be that a brief discharge passing along a 

 nerve fibre is the sign of the passage of a nervous impulse producing 

 a change in consciousness which we have learned to associate with 

 touch, while a longer discharge in the same fibre signifies pain. 

 This is not true of all touch receptors. For instance, Meissner's 

 corpuscles never give rise to pain when stimulated. The naked 

 terminations of nerve fibres arborising in the skin may, as Sherring- 

 ton suggests, act as receptors for touch and pain — -touch when the 

 stimulus is slight, and pain when it is massive. 



Pain may be produced by direct stimulation of the nervous 

 system, either by the application of induction shocks, by mecha- 

 nical force (pulling, tearing, pressing, etc.), or by chemical 

 means (drying, application of solutions, etc.), and in this case the 

 intensity of the pain depends on the number of nerve fibres in the 

 nerve trunk stimulated. The central sensory area may also 

 receive painful stimulation without the apparent intervention of 

 specialised receptors. A rise in blood pressure, for example, 

 results in a headache relieved by the administration of a vaso- 

 dilator. Pain, then, may be considered as allied to both touch and 

 pressure, but possessing the characteristic of a massiveness in the 

 electrical variations accompanying its transmission to the cortex. 



II. PHASIC-POSTURAL RECEPTORS 



4. Pressure. The receptors for pressure are more of the ijostural 

 than of the phasic type. That is, a single continued stimulation 

 produces a prolonged sensation. In fact, the sensation outlasts 

 the stimulus by quite an appreciable time. Absolute sensitiveness 

 as indicated by a sense of pressure is generally determined by 

 finding a minimum pressure necessary to evoke a minimal sensa- 



