656 A MANUAL OF PHYSIOLOGY 



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 that the second falls into the 

 muscle during the contraction produced by the first. The result 

 here is very different : traces of two contractions 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. 246). Al- 

 though 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 stimu- 

 lus acted ; and this is true even 

 when both stimuli are maxi- 

 mal. Under favourable cir- 

 cumstances, when the second 

 curve rises from the apex of 

 the first, the total height may 

 be twice as great as that of 

 FIG. 246. SUPERPOSITION OF CONTRAC- the contraction which one 

 TIONS. stimulus would have caused 



i is the curve when only one stimulus (? 7 11 )-. Jt is worthy of note 

 is thrown in ; 2, when a second stimulus *"* striated muscle has no 

 acts at the time when curve i has nearly power of summation of sub- 

 reached its maximum height. minimal stimuli each of which 



is just too weak to cause con- 

 traction. 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. 247). 



The meaning of a complete tetanus is readily grasped if, 

 beginning with a series of shocks of such rapidity that the 

 muscle can just completely relax in the intervals between suc- 

 cessive stimuli, we gradually increase the frequency (p. 711). 

 As this is done, the ripples on the curve become smaller and 

 smaller, and at last fade out altogether. The maximum height 

 of the contraction is greater than that produced by the strongest 

 single stimulus ; and even after complete fusion has been attained, 

 a further increase of the frequency of stimulation may cause the 

 curve still to rise. 



It is evident from what has been said that the frequency of 



