CUTANEOUS AND INTERNAL SENSATIONS. 267 



nerve trunks affect these fibers with especial ease. A temperature 

 of 50 to 70 C. applied to an afferent nerve will cause violent pain 

 sensations, but has no effect upon the motor nerve fibers in the same 

 trunk. Mechanical stimulation gives usually only pain sensations, 

 and the results of inflammatory changes, as in neuritis or neuralgia, 

 are equally marked. 



Localization or Projection of Pain Sensations. Under normal 

 conditions cutaneous pains are projected with accuracy to the point 

 stimulated, and it is possible' that this result is due in part at least 

 to the training acquired in connection with concomitant (epicritic) 

 pressure sensations, the latter acting as a guide or aid in the pro- 

 jection. Thus in the cases referred to above, in which a portion 

 " of the skin had lost the sense of pressure and temperature, but 

 retained that of pain, it was found that the localization was very 

 incomplete. Pain arising in the internal organs, on the contrary, 

 is located very inaccurately. The pain from a severe toothache, 

 for example, may be projected quite diffusely to the side of the face. 

 A very interesting fact in this connection is that such pains are 

 often referred to points on the skin and may be accompanied by 

 skin areas of tenderness. Pains of this kind that are misreferred 

 to the surface of the body are designated as reflected pains. It has 

 been shown by Head * and others that the different visceral organs 

 have, in this respect, a more or less definite relation to certain 

 areas of the skin. Pains arising from stimuli acting upon the 

 intestines are located in the skin of the back, loins, and abdomen 

 in the area supplied by the ninth, tenth, and eleventh dorsal 

 spinal nerves; pains from irritations in the stomach are located 

 in the skin over the ensiform cartilage; those from the heart in the 

 scapular region, and so on. The explanation offered for this 

 misref erence is that the pain is referred to the skin region that is 

 supplied from the spinal segment from which the organ in question 

 receives its sensory fibers, the misref erence being due to a diffusion 

 in the nerve centers. As Head expresses it, "when a painful 

 stimulus is applied to a part of low sensibility in close central 

 connection with a part of much greater sensibility the pain pro- 

 duced is felt in the part of higher sensibility rather than in the part 

 of lower sensibility to which the stimulus was actually applied/' 

 It is interesting that affections of the serous cavities e. g., the 

 peritoneum do not cause reflected pains or cutaneous tenderness 

 as in the case of the viscera. Another notable fact in this connec- 

 tion is the occurrence of the condition known as allochiria. When 

 from any cause one or other of the cutaneous senses is depressed 

 in a given area stimulation in this region may give sensations 

 which are referred to the symmetrical area on the other side of 

 *Head, "Brain," 16, 1, 1893, and 24, 345, 1901. 



