90 HUMAN PHYSIOLOGY 



nerve cells are kept normal; degeneration occurs in the 

 parts of the fibers which have been deprived of this 

 connection. In most nerves all the related cells are in 

 the central axis or in ganglia detached nodules of gray 

 matter near it; degeneration will therefore be periph- 

 eral to the cut and will involve all the fibers in that 

 direction. The principle of degeneration has had a 

 value for students of the nervous system. It has been 

 practicable to make carefully defined cuts in the brain 

 or the cord and by observing the changes occurring in 

 the course of some weeks afterward to show that in 

 certain bundles the governing cells have been left above 

 and in others below the injury. The animal must be 

 kept alive until the process is completed and then killed 

 for postmortem study. 



Regeneration. A nerve which has been cut and which 

 has lost its normal character throughout the course 

 from the incision to the endings may grow again. In 

 its degenerate condition it is still represented by a strand 

 of modified tissue. If there is no distinct obstacle to the 

 new development an extension of the cut fibers may be 

 effected along their original track. It is certainly re- 

 markable that several thousand fibers may be so guided 

 within their connective-tissue sheath as to make dis- 

 tant and useful connections. Within the central nervous 

 system degenerated fibers are not renewed. This sets 

 a limit to the process of recovery from injuries to the 

 brain and cord. Yet the outlook is often better than 

 might be assumed for supplementary paths are brought 

 into use, sometimes with surprising success. 



Fatigue. How the dendrites of motor cells are 

 brought under stimulation will be the subject of the 

 next chapter. At this point we shall extend somewhat 

 the ideas of fatigue previously developed. It has been 

 stated that only in laboratory experiments can we ob- 

 serve the behavior of a muscle apart from its nervous 

 connections. Muscle fatigue, pure and simple, is 

 unknown to our individual experience. There is reason 



