48 TETANIC CONTRACTIONS. [BOOK i. 



4. The return of a muscle to its former length, occupying about 

 ylfosec. 



We have given what may be considered the average duration l 

 of each phase chiefly for the sake of shewing their relative propor- 

 tions. But it must be borne in mind that the duration of a 

 contraction differs in different animals and in different muscles of 

 the same animal ; in the rabbit the more deeply coloured SQ^called 

 "red" muscles have in their contraction a longer period than 

 have the pale muscles. The duration may also differ in the same 

 muscle under different conditions; moreover the duration of the 

 several phases may vary independently. Temperature has a marked 

 effect in varying the length of the muscle-curve, a higlTtemperature 

 shortening, and a low temperature prolonging, the contraction, and 

 especially the third phase or relaxation. Fatigue also lengthens 

 the contraction as do also to a remarkable extent certain poisons 

 such as veratrin. An increase in the load which the muscle is 

 lifting, shortens the descending or return part of the curve and 

 increases the length of the latent period. All such influences will 

 be better studied when we come to speak more in detail of the 

 changes which take place in a muscle during contraction. Their 

 effects are only mentioned now in order that the reader may 

 thus early learn to conceive of even a simple muscular contraction 

 as a complex act, the several parts of which are variable, so that 

 many differing forms of a muscle-curve may be obtained under 

 different circumstances. 



Tetanic Contractions. 



If a single induction-shock be followed at a sufficiently short 

 interval by a second shock of the same strength, the first simple 



FIG. 6. TRACING or A DOUBLE MUSCLE-CURVE. To be read from left to right. 



While the muscle 2 was engaged in the first contraction (whose complete course, 

 had nothing intervened, is indicated by the dotted line), a second induction-shock 

 was thrown in, at such a time that the second contraction began just as the first 

 was beginning to decline. The second curve is seen to start from the first, as does 

 the first from the base-line. 



1 The curve described in the previous text happened to have a rather long latent 

 period, and the lengthening to be of shorter instead of longer duration than the 

 shortening. 



2 In this and the other curves of this section the tracings figured were taken 

 from frog's muscle. 



