262 THE SPECIAL SENSES. 



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* 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 

 peculiar quality of sensation. According to this view, artificial 

 stimulation, if properly controlled, of the trunks of the nerves 

 supplying the skin should be capable of bringing out these different 

 sense qualities. Experiments made with this point in view have 

 not, however, been very successful. Mechanical or electrical stimu- 

 lation of the ulnar nerve, for instance, gives usually only pain sensa- 

 tions, although if the stimulus is feeble contact sensations are 

 aroused. The method, however, is probably at fault. In the case 

 of amputated fingers or limbs a more decisive result is obtained. 

 As is well known, individuals after such operations may for many 

 years have sensations of their lost fingers or limbs. In such cases 

 the pressure in the stump of the wound acting upon the central 

 ends of the sensory fibers arouses sensations which are projected 

 in the usual way, and give the feeling that would be experienced 

 if the lost parts were still there and were stimulated in the normal 

 manner. 



* Barker, "Journal of Experimental Medicine/' 1, 348, 1896. 



