OTHER FORMS OF CONTRACTILE TISSUE 245 



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 are 

 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 summation may 

 be mechanical or physiological, the former being observed when the stimulus 

 is repeated during the decline of the excitatory process and being due 

 simply to the after-loading of a muscle by the first contraction. It is 

 best marked 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 in 

 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 

 before any contraction has occurred, a summation of stimuli. Each stimulus, 

 in fact, alters the state of the contractile tissue 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 



