126 NERVE INJURIES 



is he made aware of the stimulus? We must re- 

 member that tiny nerve twigs are to be found in 

 unexpected places ; in fasciae, tendons, and bone, 

 entering them far up the limb ; in the walls of 

 cutaneous vessels ; also that there is always a con- 

 siderable overlap of the distribution of neighbouring 

 nerves, at any rate of their finest terminals, to be 

 followed only by the microscope. The deep dis- 

 tribution both of the ulnar and radial nerves, in the 

 instance given, is wider than their cutaneous dis- 

 tribution. It is probable, though not certain, that 

 extremes of temperature and painful stimuli are 

 effective because they penetrate to the subepithelial 

 tissues.* 



We do not now refer the so-called " trophic 

 changes " to loss of innervation by special nerve 

 fibres whose sole function is to maintain the nutrition 

 of the part. The vulnerability of the parts to injury 

 or invasion by bacteria can be explained without 

 any such theory. To find the simpler explanation, 

 we have to ask how a particular part of the body is 

 able to obtain a better blood-supply at need. The 

 answer is twofold. There is a local chemical action 

 independent of nerves. A nerveless limb will show 

 hypersemia when a mustard plaster is applied. A 



* Head denies this, believing that there is a different and more 

 primitive sensory apparatus, the protopathic, detecting extremes 

 of heat and cold, and a more recently acquired sensory apparatus, 

 the epicritic, detecting the smaller ranges. He bases his opinion 

 principally on the examination of a small area in his own arm 

 after division of the radial nerve : in this area epicritic sense was 

 intact, but protopathic sense was lost. He also states that the 

 viscera possess only protopathic sense : it is, however, probable 

 that the stomach and colon have no temperature sense at all. 



