112 THE CONTRACTILE TISSUES. 



variety of circumstances, the same stimulus produces a smaller contraction, 

 i. e. y the irritability of the preparation has diminished. In other words, the 

 muscle or nerve or both have become partially "exhausted," and the 

 exhaustion subsequently increases, the same stimulus producing smaller con- 

 tractions, until at last all irritability is lost, no stimulus, however strong, 

 producing any contraction, whether applied to the nerve or directly to the 

 muscle ; and eventually the muscle, as we have seen, becomes rigid. The 

 progress of this exhaustion is more rapid in the nerves than in the muscles ; 

 for some time after the nerve-trunk has ceased to respond to even the 

 strongest stimulus, contractions may be obtained by applying the stimulus 

 directly to the muscle. It is much more rapid in the warm-blooded than in 

 the cold-blooded animals. The muscles and nerves of the former lose their 

 irritability when removed from the body, after a period varying according 

 to circumstances, from a few minutes to two or three hours ; those of cold- 

 blooded animals (or at least of an amphibian or a reptile) may, under 

 favorable conditions, remain irritable for two, three, or even more days. 

 The duration of irritability in warm-blooded animals may, however, be 

 considerably prolonged by reducing the temperature of the body before 

 death. 



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 neighboring muscular sub- 

 stance. It has been called an " uii.o-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 circumstances. 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 light 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 toward the peripheral ter- 

 minations. This centrifugal feature of the loss of irritability is often 

 spoken of as the Hitter- Valli law.. In a mammal it may be two or three 

 days ; in a frog, as many, or even more weeks, before irritability has dis- 

 appeared from the nerve-trunk. Itjis maintained in the small (and espe- 

 cially in the intramuscular) branches for still longer periods. 



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. 



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 con- 

 stant current, the decline of irritability and attendant loss of nutritive 

 power may be postponed for some considerable time. But as far as our ex- 



