308 



ADVANCED PHYSIOLOGY 



Sheath 



FIG. 161. A CROSS SECTION OF A 



NERVE 



Showing it to be a bundle of fibres held 

 in a sheath of connective tissue. 



In the spinal nerves and every nerve leaving or entering 

 the brain or cord there are very many of these minute fibres 

 running together in a bundle. It is this bundle of nerve fibres 



which is meant when the term 

 Fibre nerve is popularly used; Fig. 



161. Even in the bundle, 

 each fibre has its medullary 

 and primitive sheaths; only 

 after the fibre has entered or 

 before it leaves the brain or 

 spinal cord is the primitive 

 sheath left off. In the white 

 matter of the central nervous 

 system the fibres have unin- 

 terrupted medullary sheaths, 

 i. e. not divided into nodes 

 and internodes, and there are no external sheaths. In 

 very exceptional cases, as for example, the nerves going to 

 the nose, which is so near the brain, both medullary and 

 primitive sheaths are wanting. 



Replacement of Nerves after their Injury. Whenever a per- 

 son cuts or otherwise injures the skin or any muscle just 

 beneath it, healing begins almost at once and soon there is 

 enough new tissue formed to repair the wound completely. 

 When, however, one of the long nerve fibres which pass from 

 the cord or brain to the surface of the body is cut off, repair 

 takes place in a very different way. The two ends of the cut 

 nerve will never come together again and mend, but the part 

 of the fibre between the point of injury and the organ to which 

 it goes dies. The stump between the injured place and the 

 cord or brain often does not deteriorate at all, or only for a 

 very short distance. 



A new nerve to replace the portion of the nerve peripheral 

 to the injury is formed from material made by the cells of 

 the old primitive sheath. These increase in number along 



