106 INTRODUCTION TO GENERAL PHYSIOLOGY 



activity, we experience something which differs in quality from 

 that associated with the activity of another region. And this 

 applies down to the individual cell at the end of each nerve fibre. 



It is somewhat as if a man lives in an office in which electric 

 bells are fitted in various positions on the walls. The bells are all 

 alike, but each is connected with a different kind of factory in the 

 town. When the bell in one corner rings, the man knows that a 

 silk factory is at work ; when that in another corner rings, a brass 

 foundry starts work, and so on. But we have also to suppose that 

 the visualising power of the man is good enough to picture the 

 factories as if he were there. 



When a message comes along a nerve fibre from the foot or the 

 hand, we refer it correctly to its place of origin, although there is 

 no reason to suppose that the process in the nerve fibre itself 

 differs in the two cases. It is merely that it passes to a different 

 place in the brain. The psychological reader will recognise that 

 we are concerned with what has been called " local sign." 



We may now proceed to discuss, more or less briefly, the 

 different kinds of receptors. 



Physiologically, the most primitive and simple is the sensation 

 of pain, associated with the action of something that is likely to 

 cause actual injury. In this case there are no specialised receptors. 

 The nerve fibres come to an end between ordinary cells, and the 

 stimulus acts directly upon the nerve itself. The sensitiveness 

 is therefore not great. It would indeed be a disadvantage if it 

 were, since the muscular reactions due to pain are usually powerful, 

 and it would be undesirable to provoke them unless there were 

 actual risk of injury. The protective function of pain would be 

 defeated if innocuous contacts excited it. The skin contains nerve 

 endings of this kind, along with specialised receptors. The cell 

 layer covering the front of the eye contains no other kind of sense 

 organ, and is sensitive to pain only. 



The sense of touch is associated with special receptors 

 adjusted to be responsive to very slight degrees of deformation. 

 These receptors are localised in spots in the skin, usually around 

 hairs. Although the presence of the hairs increases the sensibility, 

 apparently by some kind of lever action, the sensation of touch is 

 still present when the hairs are removed. The whiskers of the cat 

 are extremely sensitive organs of touch, and their roots in the skin 

 are copiously supplied with nerves. The structure of the various 

 receptor organs for touch does not throw much light on the way in 

 which they act. 



Heat and Cold. If an object, applied to the skin, is at a higher 

 temperature than the skin itself, we call it warm; if at a lower 

 temperature, it is said to be cold. Like touch, there are separate 



