CUTANEOUS SENSATIONS 551 



their local sign would have to be changed in accordance with the 

 changed conditions, before the pea would be perceived in its true 

 state as single. 



THE PAIN SENSE 



When the pressure of a hard object on the skin is increased beyond 

 that necessary to evoke a tactile sensation, at a certain pressure the 

 quality of sensation changes and it becomes painful. For the evolu- 

 tion of the race as well as for the preservation of the individual this 

 pain sense is all- important ; it is the expression in consciousness of the 

 reflexes of self-preservation which can be evoked in the spinal animal 

 by stimuli which are nocuous, i.e. calculated to do actual damage to the 

 tissues of the body. Thus when a sharp point is pressed on the skin 

 the sensation becomes painful just before the pressure is sufficient to 

 cause penetration. The so-called trophic lesions which occur in parts 

 devoid of sensation are determined for the most part by the lack of 

 the pain sense and the consequent failure of the preservative reflexes 

 of the part. It is remarkable that pain may result from changes in 

 organs which are devoid of ordinary sensibility. Thus the intestine 

 may be cut, sewn, or handled without arousing any sensation what- 

 soever. A strong contraction of the muscular wall or increased dis- 

 tension of the gut will, however, evoke a griping pain. In the same 

 way the ureters, which are devoid of sensation, can give rise to excru- 

 ciating agony when they are contracted firmly on a retained calculus. 



We are accustomed to distinguish many different qualities of pain, 

 but on analysis it will be found that these qualities depend on the 

 nature of the sense-organ which is simultaneously stimulated. Thus 

 a burning pain denotes simultaneous stimulation of the pain sense as 

 well as of the nerve-endings to the warm spots. A throbbing pain 

 results when the vessels of the part are dilated and the part is tense 

 with effused lymph, so that each pulse of the vessels causes an exacer- 

 bation of the painful stimulation and perhaps also stimulation of the 

 tactile end organs. 



The sense of pain has often been ascribed to over-maximal stimula- 

 tion of any form of sensory nerve. Although it is true that over- 

 stimulation of the auditory or optic nerve by a loud sound or a bright 

 light may be extremely unpleasant, the sensations evoked do not 

 partake of the characters of painful sensations such as would be 

 produced by pricking or burning the skin. Moreover a careful investi- 

 gation of the sensory points on the skin brings out the fact that there 

 are, besides the tactile and temperature spots, other spots from which 

 only painful sensations can be evoked. We' have seen already that 

 over-stimulation of a touch spot does not, as^a matter of fact, cause 

 pain. The pain spots which are distributed among the touch and 



