96 HUMAN PHYSIOLOGY 



certain localities they produce sensations such as tickling, 

 tingling, &c., and generally lead to an endeavour to remove 

 the abnormal stimulus. 



Those modifications of consciousness produced by this 

 mechanism of common sensibility may be small or great 

 according to the strength of the stimulus, and according to 

 the state of the central nervous system, and when excessive 

 the sensation produced is called pain. All pain, since it 

 means a change in our consciousness, is metaphysical. There 

 is not such a thing as " physical pain." The fatigue and 

 other sequences to any kind of pain are frequently cited as 

 proofs of the influence of mind on the body. But we have 

 no right to assume that they are caused by the pain rather 

 than by the physical disturbances in the nervous system of 

 which the pain is an accompaniment. It must be recognised 

 that pain is a purely relative term, and that conditions which 

 in one individual will cause pain will not cause it in another, 

 while stimuli which will produce what are called painful 

 sensations when the nervous system is debilitated may give 

 rise to sensations not considered as painful when the nervous 

 system is normal. 



The mechanism of common sensibility and pain is not 

 acted on by the same stimuli in all parts of the body. The 

 mouth and throat are the parts to which the sensations are 

 referred in abstinence from fluids, the stomach in absence of 

 food. The intestine appears to give rise to sensation only 

 when abnormally stimulated. In the skin the mechanism of 

 common sensibility is so closely associated with the mechanism 

 of the tactile and temperature senses that it is difficult to 

 differentiate them. Abnormal stimulation of the skin pro- 

 duces painful sensations very readily, while similar changes 

 in other tissues e.g. muscles cause no modification of 

 consciousness. 



The nerve channels by which the changes producing 

 common sensibility and pain are transmitted to the central 

 nervous system appear to be distinct from those connected 

 with the tactile and other senses, since common sensibility 

 may persist while the tactile sense is lost. 



