NERVE-DEGENERATION AND NERVE-REGENERATION. 635 



and later on diminution of irritability through exhaustion. Neverthe- 

 less the nerve exhibits extraordinary resistance to various stimuli. It 

 may not be exhausted even after irritation continued for hours. 



The enfeeblement and finally the cessation of muscular contraction after 

 long-continued stimulation of the motor nerve connected with the muscle are due 

 to exhaustion of the muscle and not of the nerve. If while a nerve is being stimu- 

 lated the muscle is prevented from contracting, by rendering the nerve incapable 

 of conducting at a point distal to the site of stimulation (by anelectrotonus or 

 curare) , it will be found that even after twelve hours of constant irritation of the 

 nerve, the muscle can again be made to contract if this obstruction (blocking of 

 the nerve) be removed. Also the observation that the negative variation in the 

 nerve-current in an irritated nerve continues for a long time is interpreted in the 

 same way. 



The recovery of nerves takes place at first slowly, then somewhat 

 more quickly, and finally again more slowly. Should recovery not take 

 place in the first half -hour in the frog, after long, intense irritation, the 

 nerve does not recover at all. 



Long-continued inactivity diminishes the irritability to the point 

 of complete abolition. 



The characteristic example of this is furnished by the degeneration of nerves 

 after amputation of an extremity. Not only the sensory nerves to the cutaneous 

 area, etc., removed, but also the motor nerves to the muscles removed, undergo 

 atrophy, and also their continuations in the spinal cord exhibit atrophic changes. 

 The degeneration of the optic nerve after extirpation of the eye and of the auditory 

 nerve after that of the internal ear is considered on pp. 679 and 699. 



Excised nerves preserve for a time, as does muscle, their func- 

 tional activity. At first the end-apparatus of the nerve degenerates ; 

 then, in the case of motor nerves, the muscle; and finally the nerve 

 itself. 



Nerve-fibers are capable of maintaining their normal nutrition only 

 when they are in uninterrupted connection with their trophic center, 

 which controls the nutritive processes. If, however, the nerve within 

 the otherwise normal body is separated from its nutritional center, as 

 by section or crushing, it loses its irritability in a short time and the per- 

 ipheral end undergoes fatty degeneration, which begins in warm-blooded 

 animals in the course of from tour to six days, in cold-blooded animals 

 after a longer interval. 



The irritability of the nerve under these conditions the so-called reaction of 

 degeneration is discussed on p. 672. The degeneration after section of the roots 

 of the spinal nerves is described on p. 716. 



In the otherwise intact body both extremities at the point of division 

 undergo traumatic degeneration in from one to two days in frogs, as 

 a result of which the white substance of Schwann and the axis-cylinder 

 can no longer be distinctly differentiated. This degeneration extends, 

 however, only to the next node of Ranvier. Later, so-called fatty 

 degeneration takes place simultaneously in the entire peripheral portion. 



Fatty degeneration of nerves begins by a breaking up of the myelin (Fig. 

 223, .4), which later becomes transformed into drop-like masses (B). The axis- 

 cylinder also swells up and disintegrates (seventh day) (Q. The nuclei in the 

 sheath of Schwann become swollen and proliferate by mitosis (up to the tenth 

 day) (D). According to Ranvier, it is this nuclear proliferation and that of the 

 protoplasm or neuroplasm lining the sheath of Schwann that first cause disinte- 

 gration of the myelin and the axis-cylinder and that subsequently increase to such 



