144 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 however 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, arid 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 has 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. The medulla suffers 

 changes similar to those seen in nerve fibres after removal from the 

 body; its double contour and its characteristic indentations be- 

 come more marked, it breaks up into small irregular fragments, or 

 drops. Mingled with the fat particles of the medulla are seen 

 small masses of proteid material which appear to be derived 

 from the protoplasm around the nuclei. Meanwhile the axis 

 cylinder also breaks up into fragments, and the nuclei of the 

 .neurilemma divide and multiply. The fatty constituents sub- 

 sequently decrease in amount, the proteid material increasing 

 or not diminishing, and thus the contents of the neurilemma 

 between each two nodes is reduced to a mass of proteid material, 

 in which the fragments of the axis cylinder can no longer be 

 recognised. This mass which still retains some fat globules, is 

 studded with nuclei. If no regeneration takes place these nuclei 

 with their proteid bed eventually disappear. 



In the central portion of the divided nerve similar changes may 

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

 the nerve usually remains in a normal condition. 



