146 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, 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 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 first 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 then breaks up into small irregular frag- 

 ments, or drops, and, as shewn by the behaviour towards staining 

 reagents, becomes somewhat altered in its chemical nature. The 

 axis cylinder also breaks up into fragments. Meanwhile the nuclei 

 of the neurilemma divide and multiply, and with their multiplica- 

 tion, a great increase of the protoplasmic material surrounding 

 them appears to take place ; this at least seems to be the origin of 

 a conspicuous bed of protoplasmic-looking substance in which the 

 fragments of the medulla and of the axis-cylinder are imbedded. 

 These fragments becoming more and more altered in chemical 

 nature, are now absorbed, the protoplasmic-looking material in- 

 creasing or not diminishing. 



The neurilemma collapses, and so the nerve fibre is reduced to 

 a strand of protoplasmic material studded with nuclei and con- 

 taining drops or globules of fat which are the remains of the 

 medulla, the fragments of the axis-cylinder having wholly dis- 



