CtTTANEOUS AND INTERNAL SENSATIONS. 263 



to the skin, and for cutaneous pain at least the evidence, as stated 

 above, is very strongly in favor of the view that there exists a special 

 set of fibers which have a specific energy for pain. All recent ob- 

 servers agree that the pain sense has a punctiform distribution in 

 the skin, the pain points being even more numerous than the pres- 

 sure points. The threshold stimulus of these points in various 

 regions may be determined by von Frey's stimulating hairs, and 

 experiments of this kind show, as we should expect, that it varies 

 greatly. The cornea, for instance, gives sensations of pain with 

 much weaker stimuli than in the case of the finger tips. In general, 

 however, the threshold stimulus is much higher for the pain than 

 for the pressure points. Histological examination of the pain points 

 indicates that there is no special end-organ, the stimulus taking 

 effect upon the free endings of the nerve fibers. Any of the usual 

 forms of artificial nerve stimuli may affect these endings if of suf- 

 ficient intensity, and, as is well known, stimuli applied to sensory 

 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 pressure 

 stimuli, the latter acting as a guide or aid in the projection. 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 misref erred to the surface of the body are desig- 

 nated 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 

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



