756 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



nemius. The wave of contraction, which in frogs' striped muscle lasts 

 only about 0-07 second at any point, may last a second in- the forceps 

 muscle of the crayfish, though only half as long in the muscles of the tail. 

 In the muscles of the tortoise the contraction is also very slow. The 

 muscles of the arm of man contract more quickly than those of the leg. 

 Summation of Stimuli and Superposition of Contractions. Hitherto 

 we have considered a single muscular contraction as arising from a 

 single stimulus, and we have assumed that the muscle has completed 

 its curve and come back to its original length before the next stimulus 

 was thrown in. We have now to inquire what happens when a second 

 stimulus acts upon the muscle during the contraction caused by a first 

 stimulus, or during the latent period before the contraction has actually 

 begun; and what happens when a whole series of rapidly-succeeding 

 stimuli are thrown into the muscle. 



First let us take two stimuli separated by a smaller interval than 

 the latent period (p. 745). If they are both maximal i.e., if each by 

 itself would produce the greatest amount of contraction of which the 

 muscle is capable when excited by a single stimulus the second has 

 no effect whatever; the contraction is precisely the same as if it had 



never acted. But if they are less 

 than maximal, the contraction, 

 although it is a single contraction, 

 is greater than would have been 

 due to the first stimulus alone; in 

 other words, the stimuli have been 

 summed or added to each other 

 during the latent period, so as to 

 produce a single result. 



Next let us consider the case of 

 two stimuli separated by a greater 

 interval than the latent period, so 

 Fig. 266 -Superposition of Contractions. that the second falls l nto the 

 i is the curve when only one stimu us j during the contraction pro- 



is thrown in; 2, when a second stimulus ,, 9- 



. acts at the time when curve i has nearly duced by the first The result here 

 reached its maximum height. 1S ver Y different : traces of two con- 



tractions appear upon the muscle- 

 curve, the second curve being that which the second stimulus would have 

 caused alone, but rising from the point which the first had reached at the 

 moment of the second shock (Fig. 266). Although the first curve is 

 cut short in this manner, the total height of the contraction is greater 

 than it would have been had only the first stimulus acted ; and this is 

 true even when both stimuli are maximal. Under favourable circum- 

 stances, when the second curve rises from the apex of the first, the total 

 height may be twice as great as that of the contraction which one 

 stimulus would have caused (p. 816). It is worthy of note that striated 

 muscle has no power of summation of subminimal stimuli each of which 

 is just too weak to cause contraction. No matter how rapidly they are 

 thrown in, the muscle remains at rest. It is otherwise with smooth 

 muscle. Stimuli which are singly ineffective cause contraction when 

 repeated. 



Tetanus. Not only may we have superposition or fusion of two 

 contractions, but of an indefinite number; and a series of rapidly 

 following stimuli causes complete tetanus of the muscle, which 

 remains contracted during the stimulation, or till it is exhausted 

 (Fig: 267). 



