258 THE SPECIAL SENSES. 



carefully by von Frey.* To determine the location of the pressure 

 points he used fine hairs of different diameters fastened to a wooden 

 handle. The cross-areas of these hairs are determined by measure- 

 ments under the microscope, and the pressure exerted by each is 

 measured by pressing it upon the scale pan of a balance. The 

 quotient of the pressure exerted divided by the cross-area of the 



hair in square millimeters, ^"^-2, reduces the pressure to a uniform 



unit of area. For the pain points fine needles may be employed 

 or stiff hairs similar to those used for the pressure points. From 

 the experiments made there seems to be no doubt that each of 

 the four cutaneous senses has its own spots of distribution in the 

 skin, those for pain being most numerous and those for warmth 

 the least numerous. There is some reason for believing also that 

 the nerve endings mediating the pain sense lie most superficially 

 in the skin and those for the warm sense the deepest. 



Specific Nerve Energies of the Cutaneous Nerves. Many 

 attempts have been made to determine whether the doctrine of 

 specific nerve energies applies to these cutaneous senses; that is, 

 whether each sense has its own nerve fibers capable of giving only its 

 own quality of sensation. The evidence, on the whole, is favorable 

 to this view. According to some observers, electrical or mechanical 

 stimulation of the different points calls forth for each its character- 

 istic reaction. Donaldson has found that cocain applied to the 

 eye or throat destroys the senses of pain and pressure, but leaves 

 those of heat and cold, which again supports the view of separate 

 fibers for each sense. In addition there are a number of interesting 

 pathological cases which point in the same direction. In some 

 lesions of the cord syringomyelia, for instance the senses in the 

 skin of the parts below are dissociated, that is, there may be loss 

 of pain and temperature in a certain area with a retention of the 

 pressure sense. a fact which indicates that these senses have 

 separate paths and therefore separate nerve fibers. Still more 

 interesting cases of dissociation are reported as the result of the 

 compression of peripheral nerve trunks. Thus, Barker f describes 

 his own case, in which, as the result of the pressure of a cervical 

 rib upon some of the cords of the brachial plexus, there was a region 

 in the arm lacking in the pressure and temperature senses, but retain- 

 ing the sense of pain. He quotes other cases in which the reverse 

 dissociation occurred, pressure sense alone remaining. The simplest 

 explanation of these facts is the view that each pressure, pain, 

 warm, and cold spot is supplied by its own nerve fiber, and that 

 each, when stimulated, reacts, if it reacts at all, only with its own 



* Von Frey, "Konigl. Sachsischen Gesellschaft der Wissenschaften. Math.- 

 phys. Klasse," 1894-95-96. 



t Barker, "Journal of Experimental Medicine," 1, 348, 1896. 



