CHAP, ii.] THE CONTRACTILE TISSUES. 91 



If with some thin body a sharp blow be struck across a muscle which 

 has entered into the later stages of exhaustion, a wheal lasting for 

 several seconds is developed. This wheal appears to be a contraction 

 wave limited to the part struck, and disappearing very slowly, without 

 extending to the neighbouring muscular substance. It has been called 

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

 ordinarv 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 exJiausting 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 various events. 

 We have already seen (p. 78) how the constant current produces 

 the variations in irritability known as katelectrotonus and anelec- 

 trotonus. We have now to study the effect of more 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 Bitter- 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, a separation apparently taking place between its proteid and 

 its fatty constituents. The latter are soon absorbed, but the 

 former remain for a longer time within the sheath of Schwann, 

 being in some cases scarcely, if at all, to be distinguished from the 



