126 DEGENERATION OF NERVES. [BOOK i. 



an ' idio-muscular ' contraction, because it may be brought out even 

 when ordinary stimuli have ceased to produce any effect. It may how- 

 ever be accompanied at its beginning by an ordinary contraction. It 

 is readily produced in the living body on the pectoral and other muscles 

 of persons suffering from phthisis and other exhausting diseases. 



This natural exhaustion and diminution of irritability in 

 muscles and nerves removed from the body may be modified both 

 in the case of the muscle and of the nerve, by a variety of circum- 

 stances. Similarly, while the nerve and muscle still remain in the 

 body, the irritability of the one or of the other may be modified 

 either in the way of increase or of decrease by certain general 

 influences, of which the most important are, severance from the 

 central nervous system, and variations in temperature, in blood 

 supply, and in functional activity. 



The Effects of Severance from the Central Nervous System. 

 When a nerve, such for instance as the sciatic, is divided in 

 situ, in the living body, there is first of all observed a slight 

 increase of irritability, noticeable especially near the cut end ; but 

 after a while the irritability diminishes, and gradually disappears. 

 Both the slight initial increase and the subsequent decrease begin 

 at the cut end and advance centrifugally towards the peripheral 

 terminations. This centrifugal feature of the loss of irritability is 

 often spoken of as the Ritter-Valli law. In a mammal it may be 

 two or three days, in a frog, as many, or even more weeks, before 

 irritability b as disappeared from the nerve trunk. It is maintained 

 in the small (and especially in the intramuscular) branches for 

 still longer periods. 



This centrifugal loss of irritability is the forerunner in the 

 peripheral portion of the divided nerve of structural changes 

 which proceed in a similar centrifugal manner. In the central 

 portion of the divided nerve these structural changes may be 

 traced as far only as the next node of Ranvier. Beyond this 

 the nerve usually remains in a normal condition. Such a degen- 

 eration may be observed to extend down to the very endings of 

 the nerve in the muscle, including the end-plates, but does not 

 at first affect the muscular substance itself. The muscle, though 

 it has lost all its nervous elements, still remains irritable towards 

 stimuli applied directly to itself : an additional proof of the 

 existence of an independent muscular irritability. 



If the muscle thus deprived of its nervous elements be left to 

 itself its irritability, however tested, sooner or later diminishes ; but 

 if the muscle be periodically thrown into contractions by artificial 

 stimulation with the constant current, the decline of irritability 

 and attendant loss of nutritive power may be postponed for some 

 considerable time. But so far as our experience goes at present 

 the artificial stimulation cannot fully replace the natural one, and 

 sooner or later the muscle like the nerve suffers degeneration, loses 



