226 PHYSIOLOGY 



The actual duration of the shortening at any given point is neces- 

 sarily smaller than that of the whole muscle, and amounts in frog's 

 muscle to only 0-05-0-09 sec., about half the duration of the 

 contraction of a whole muscle of moderate length. The length of the 

 wave is obtained by multiplying the rate of transmission by the 

 duration of the wave at any one point. It varies therefore in frog's 

 muscle between 3000 X -05 (= 150) and 4000 X -09 (= 360) milli- 

 metres. Thus the muscle fibres in the frog are much too short to 

 accommodate the whole length of the wave, and the contraction of 

 the whole muscle must be made up of the summated effects of the 

 contraction wave as it passes from point to point. Hence the longer 

 the muscle, the more must the contraction be lengthened by the time 

 taken up in propagation from one end to another. 



SUMMATION OF CONTRACTIONS 



If a muscle or its nerve be stimulated twice in succession so that 

 the second stimulus becomes effective before the state of activity due 

 to the first stimulus has come to an end, we get a combination of the 



FIG. 61. Muscle curves showing summation of stimuli, r and r' t the points 

 at which the stimuli were sent into the nerve. From the first stimulus 

 alone the curve a b c would be obtained. From r' the curve def is 

 obtained. These two curves are summated to form the curve aghik 

 when both stimuli are sent in at the interval r i'. 



effects of the two stimuli, and the resulting contraction of the muscle 

 is as a rule greater than that which can be evoked by a single stimulus. 

 If the interval between the two stimuli is so far apart that the second 

 becomes effective just as the contraction due to the first has commenced 

 to die away, the second contraction seems to start from the point to 

 which the muscle has been raised by the first (Fig. 61). If the second 

 stimulus becomes effective at the height of the first contraction, the 

 shortening of the muscle may be almost doubled. By repeating these 

 stimuli the contraction may be made three or four times as extensive 

 as that due to a single maximal stimulus. This increase in height 

 due to summation is best marked when the muscle has to overcome 

 the resistance of a considerable load. If the muscle is extremely 

 lightly loaded, the contraction evoked by a single stimulus may be 

 as high as that which can be brought about by repeated stimuli. 

 The phenomenon of summation is due to the fact that by the first 



