210 THE HUMAN BODY. 



After complete section of the nerve-trunk supplying a 

 region of the Body that region is for a time paralyzed, but 

 feeling and the power of movement may return to it. It 

 used to be thought that in such cases the divided nerve-fibres 

 grew together again. Such is not the case : all those parts of 

 the fibres which have been cut oif from their centres com- 

 pletely disappear, and when function is restored it is by the 

 formation of new nerve-fibres around outgrowths from the 

 cut ends of those parts of the fibres still united to their cen- 

 tres, whether these be in brain, spinal cord, spinal ganglia, or 

 elsewhere. 



Nerves, as we have seen, often give fibres to one another 

 by means of uniting branches, as in various plexuses and 

 elsewhere; and when a nerve-branch may contain fibres de- 

 rived from some one of two or more original trunks which 

 have communicating branches, it is often of importance to 

 determine in which original trunk its fibres left the brain or 

 spinal cord. In such cases the determination may often be 

 made by dividing one of the possible sources of origin and 

 after a few days examining the branch for degenerated 

 fibres, which are easily recognized by the microscope. If 

 such are found, then they left the centre in the divided trunk; 

 if not, the branch gets no fibres from that trunk. This 

 method of tracking the nerve-fibres of a given original trunk 

 to their final distribution in one or more of many possible 

 branches is known as the Wallerian method. Instances of 

 its application will be given in later chapters. 



