494 PHYSIOLOGY 



position in contact with a pea, if the eyes are shut, we should at once say 

 that two peas lay under the fingers. This is especially marked if the pea 

 be rolled between the fingers. The two sides of the fingers which come in 

 contact with the pea usually touch two different objects, and these parts 

 of the skin would have to be re-educated, i.e. 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 



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

 Jfnecessary to evoke a tactile sensation, at a certain pressure the quality 

 (of sensation changes and it becomes painful. For the evolution 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 

 I 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 

 whatsoever. 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 excruciating 

 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 and 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 exacerbation of the painful stimula- 

 tion 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 investigation of the sensory points on the skin 



I 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 

 temperature spots are insensitive to a low grade of stimulus. As the 



