GENESIS OF TETANUS 63 



rate of stimulation was four per second, and it is seen that each 

 contraction is distinct. On measuring the heights of the twitches 

 it will be found that the second is a little higher than the first, and 

 the third a trace higher than the second, the last four ail being of 

 the same height. This ' staircase ' effect, very slightly marked in this 

 particular instance, is usually to be observed, especially if the pre- 

 paration be very fresh and has not been injured during its removal. 

 It shows that by a contraction a muscle is brought into such a 

 state that an immediately following stimulus of the same strength 

 evokes a more powerful response. That it is a question of increased 

 capacity of performing work rather than an increase of excitability 

 is indicated by the fact that it is to be observed when maximal 

 stimuli are employed. 



In curve 2 the rate of stimulation was increased to ten per 

 second. "We see that the first contraction was nearly complete 

 before the second stimulus led to a fresh contraction. The lever did 

 not reach the zero abscissa line, and some summation occurred. The 

 third stimulus effected a contraction before relaxation was complete, 

 and a fresh summation occurred ; and so on for the later contractions, 

 though after the fourth the maximum height of contraction did not 

 increase. 



In curve 3 the rate of stimulation was further increased to sixteen 

 per second, and a much greater summation of effect is recorded. The 

 second apex lies well above the first, the third above the second, but to 

 a less extent, and the fourth above the third, though here the increase 

 of shortening was only slight. From this the apices remained at practi- 

 cally a uniform level. An interesting point is to be noticed in this 

 curve, viz. that the lowest points of the undulations of the curve 

 become progressively at a higher level as stimulation proceeds. This 

 effect is due to fatigue brought about by the number of contractions 

 the muscle has been made to give. One of the effects of fatigue (see 

 chap, v.) is found to be a marked slowing of the rate of relaxation, 

 so that as this was produced in this instance the later stimuli began 

 to produce their effects on the muscle before relaxation had proceeded 

 quite as far as in the preceding contraction. 



In curve 4 the rate was increased to 18 - 5 per second, and in 5 to 

 22 per second. The curves show practically the same points as those 

 already described for 3. The only difference, apart from the lesser 

 excursion of the oscillations, is that there is more summation, and 

 therefore both upper and lower apices are at a higher level than in 

 the preceding curves. 



Curve 6 shows a similar condition, though carried to a further 

 degree. The line joining the upper apices is approximated to that 



