OTHER FORMS OF CONTRACTILE TISSUK , 



moistened with normal saline, thus allowing the current to leave the contractile tissues 

 anywhere along the ureter, we get the same aberrant results of stimulation as i 

 obtained with the intestine. 



In voluntary muscle, if one stimulus follows another at an interval 

 which is not too large, a summated contraction is 'produced which is 

 ) greater in amplitude than that due to a single stimulus. This summa- 

 tion may be mechanical or physiological, the former being observed 

 w 1 1 enjihe stimulus isjy pptttnd dnri^th^dec)jnq of the excitatory jgrojcess 

 and bemgJue^iB^y-be4^ after-loading oiajnnacleJiy f.] J?"rt con traction. 

 It is bestmarked when the muscle is heavily loaded. If, however, the 

 stimuli be sent in at sufficiently short intervals so that two stimuli fall 

 within the period of rise of contractile stress, an increased height of 

 contraction is obtained under all conditions, and under isometric 

 conditions the tension developed is greater than that with a single 

 stimulus. If the interval between two stimuli be so short that the 

 second falls within what we have called the refractory period due to 

 the first stimulus, no summation is obtained, the second stimulus being 

 ineffective. 



In the slow contraction of involuntary muscle we could hardly expect 

 mechanical summation to come into play. Most types of this tissue show, 

 however, the true summation, i.e. the increased liberation of energy due 

 to repetition of the stimulus during the rise of the excitatory condition. 

 As might be expected the refractory period is also longer in involuntary 

 muscle, since all the processes- of this muscle are slowed in comparison with 

 those of voluntary muscle., In certain types of tissue, and especially jjx 

 heart muscle, the refractory period lasts during the whole of the period 

 of contraction. During this time therefore a second shock will be ineffective. 

 As the contraction dies away the muscle fibre gradually recovers its sus- 

 ceptibility to stimulation, but it does not recover its full irritability until 

 it has entirely relaxed. On this account it is impossible to obtain summa- 

 tion in or to tetanise heart muscle, the application of interrupted currents 

 to this tissue producing only a series of rhythmic contractions. 



In all involuntary muscle we may observe summation of the effects of 

 stimuli even when the individual stimuli are insufficient to produce any 

 excitation. Thus in a muscle such as the retractor penis, we may find a 

 strength of induction shock which, applied singly, is just insufficient to evoke 

 any response. If, however, the shocks are repeated at intervals of a second, 

 it will be found that the first three or four stimuli are ineffective and then 

 the muscle enters into a contraction which increases with each succeeding 

 stimulus until it has attained its maximum. There is thus summation 

 fee/ore any contraction has occurred, a summation of stimuli. Each stimulus, 

 in fact, alters the state of the contractile tissus and makes it more ready 

 to respond to the next stimulus, so that the stimuli become more and more 

 effective. If time is allowed for the muscle to relax between successive 

 stimuli, this summation is evidenced by a continually increasing height 

 of contraction, the so-called ' staircase.' The same initial increase of 



