282 THE SPECIAL SENSES. 



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 sepa- 

 rate paths and therefore separate nerve-fibers.* Still more inter- 

 esting cases of dissociation are reported as a result of the com- 

 pression of peripheral nerve trunks. Thus, Barker t 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. 

 The Temperature Senses. The main facts regarding the 

 distribution of heat and cold spots have been determined, but 

 in most of the experiments on record no distinction was made 

 between protopathic temperature sensations and those mediated 

 by the epicritic temperature nerves. It is difficult to adapt 

 the older descriptions to this newer terminology, but when not 

 otherwise specifically stated it may be assumed that the ep.icritic 

 system is referred to. In general, the cold spots are more 

 numerous than the warm spots, and react more promptly to 

 their adequate stimulus. The threshold stimulus varies in 

 different parts of the skin, the tip of the tongue requiring the 



* For many interesting cases of dissociation due to spinal lesions see Head, 

 "Brain," 1906,537. 



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



