3 8 



THE PROPERTIES OF STRIPED MUSCLE. 



sufficient for the completion of the contraction. 1 The question, however, 

 whether, in the rise of tetanus, we can distinguish between the sum- 

 mation of excitations and the summation of their effects, requires further 

 experimental investigation. 



Summation of inadequate stimuli. It was long ago observed by 

 Helmholtz that when an instantaneous stimulus, which in itself is incapable 

 of evoking a maximal response, is followed during the period of latency by a 

 second of the same strength, a maximal response may be obtained. If the 

 time-interval between the two stimuli is sufficiently short (less than ^J^ second), 

 there may, as we have already seen, be an increase of effect, even when the 

 stimuli are maximal (see Fig. 209). Again, a succession of stimuli of sufficient 

 frequency may be effectual, although each stimulus when alone is so weak as 

 to produce no response. An instance of this is given in Fig. 212, which 



represents a tracing made by Kichet 2 

 with the claw-muscle of the cray- 

 fish. Seven stimuli, each inadequate 

 in itself to produce an effect, were sent 

 through the muscle in succession. 

 The first three gave rise to no response, 

 the fourth to a very slight one, while 

 the fifth, sixth, and seventh evoked 

 contractions each stronger than that 

 of its predecessor. 



Such summation of submaximal 

 stimuli can be much more easily 

 observed in slowly contracting skeletal 

 muscles, such as the claw-muscle of 

 cray-fish, than in quickly contracting 

 ones. The cardiac muscle of the frog, 

 when stimulated by induction currents 

 which singly are of insufficient strength 

 to produce an effect, responds when 

 these are repeated with sufficient fre- 

 quency, the response being slower in 

 occurring the less the frequency. 

 Thus Engelmann found that in the 

 bulb of the aorta 3 a frequency of 

 fifty inadequate stimulations per 

 second produced a response after a 

 delay of one second, a frequency of 



four per second after two and a half seconds. In smooth muscle even strong 



single induction shocks are often inadequate to produce a response, at the 



same time that tetanisation with comparatively weak stimuli may be effectual. 4 



We must distinguish between instances such as those above referred to, 



1 From Kaiser's observations, we have learned that at the moment of culmination of 

 the electro-chemical response to a maximal instantaneous stimulus (Case 1), a muscle, even 

 when after-loaded, tends to assume the length X, whereas, under the conditions of v. Frey's 

 experiment (Case 2), it shortens to its eventual length in tetanus A. There is in reality no 

 contradiction. In Case 2 the contraction culminates immediately after it begins, i.e. at 

 a time when, as Bernstein has lately shown, the process by which chemical energy is 

 transformed into muscular force is at its maximum. In Case 1 that process is already 

 declining at the moment that its mechanical effect is at maximum. This may be expressed 

 by saying that it is for want of time that the mechanical effect of a single stimulus falls 

 short of that of repeated excitation. See Kaiser's criticism of v. Frey's interpretation of 

 his observation, loc. cit., p. 367. 



2 "Physiol. des muscles et des nerfs," Paris, 1882. 



3 Arch.f. d. ges. Physiol., Bonn, 1882, Bd. xxix. S. 453. 

 'Engelmann, ibid., 1870, Bd. iii. S. 280. 



FIG. 212. Muscle curve, showing the effect 

 of a succession of stimuli, each one of 

 which alone was inadequate to produce 

 contraction. Rlchet. 



