250 INTRODUCTION TO NEUROLOGY 



a sensation of the quality typical for the organ in question, to 

 which there is added an agreeable or disagreeable quality which 

 may be very pronounced, the disagreeable quality not being 

 painful in the ordinary sense of that term. This mixed quality 

 of normal sensations is illustrated by certain odors and savors, 

 and on the agreeable side by certain sensations of tickle and 

 warmth. Finally, some ideational processes have an agreeable 

 or disagreeable quality, and these, in turn, are very intimately 

 related with the emotions and with esthetic and appreciative 

 functions of the most complex psychic sort, as well as with ques- 

 tions of habitual emotional attitude and temperament. 



The superficial parts of the body which are more directly ex- 

 posed to traumatic injury are, in general, more sensitive to pain 

 than are the deeper parts, and painful stimuli here can be more 

 accurately localized. In some parts, like the conjunctiva of the 

 eyeball, where very slight irritation may seriously interfere 

 with the function, very gentle stimulation gives rise to acute 

 pain, and no other sensory quality may be present. 



Surgeons find that the brain membranes are sensitive to 

 mechanical injury, especially to stretching or pulling. The brain 

 substance itself, however, is quite insensitive to pain from either 

 mechanical or chemical stimulation. The deeper viscera of the 

 thorax and abdomen are insensitive to pinching, cutting with a 

 sharp instrument, or other mechanical, chemical, or thermal 

 stimuli, though they are sensitive to pains arising from internal 

 disorders, as in colic (p. 243). The visceral portions of the 

 pleural and peritoneal membranes are insensitive to pain, but 

 their parietal portions, forming the innermost layer of the body 

 wall, are sensitive, and these pains can be accurately localized 

 (Capps). 



From these considerations it appears that pain is an adaptive 

 function which is present only where it is of value to give warn- 

 ing of noxious influences liable to injure the body unless re- 

 moved. (See the excellent discussion by Sherrington in 

 Schafer's Physiology, vol. ii, pp. 965-1001.) 



Pains of this sort are physiologically similar to other extero- 

 ceptive sensations, that is, they have a definite localization and 

 are externally projected like other somatic sensations. But 

 other pains and discomforts (especially those related to the 



