538 



DEFENERATION OF NERVE-FIBRES. 



3. Continued inaction of a nerve diminishes, and may ultimately abolish the 

 excitability. 



Thus, the central ends of divided sensory nerves, after amputation of a limb, lose their 

 excitability, although the nerves are still connected with the central nervous system, because 

 the end-organs through which they were normally excited have been removed. 



4. Separation from their Nerve-Centres. The nerve-fibres remain in a con- 

 dition of normal nutrition, only when they are directly connected with their 

 centre, which governs the nutritive processes within the nerve. If a 

 nerve within the body be separated from its "nutritive centre" either 

 by section of the nerve or compressing it within a short time it loses its 

 excitability, and the peripheral end undergoes fatty degeneration, which 

 begins in four to six days in warm-blooded animals, and after a long 

 time in cold-blooded ones (Joh. Miiller). See also the changes of the 

 excitability during this condition, the so-called " Reaction of degener- 

 ation " ( 339). If the sensory nerve-fibres of the root of a spinal nerve 

 be divided on the central side of the ganglion, the fibres on the peripheral 

 side do not degenerate, for the ganglion is the trophic or nutritive centre 



ft 



sa 



Fig. 377. 

 Degeneration and regeneration of nerves. A, sub-division of the myelin ; B, further disintegra- 

 tion thereof (osmic acid staining) ; C, interruption of the axial cylinder, which is 

 surrounded with the broken-up myelin ; D, accumulation of nuclei, with the remainder 

 of the myelin in a spindle-shaped fibre ; E, a new nerve-fibre, with a new sheath of 

 Schwann, sn, within the old sheath of Schwann, sa ; F, a new nerve-fibre passing in a 

 curved course through an old nerve-fibre sheath. 



for the sensory nerves; but the fibres still in connection with the cord degenerate 

 ( Waller). 6 



[WaJlerian Law of Degeneration. If a spinal nerve be divided, the peripheral 



