ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 129 



" This then affords a simple explanation of the fact, which is 

 easy to confirm, that twitches may be elicited from a muscle that 

 is already in steady and uniform contraction, as follows indeed 

 in a great number of cases " (Griitzner). 



Upon this assumption, which, as it seems to us, emphasises 

 one of the most essential and important factors that comes into 

 play in tetanus contractions, " a tetanus remains discontinuous 

 and unstable as long as the twitches of the pale fibres can be 

 superposed upon the contraction of the red. But if the red have 

 shortened to their maximum, the entire muscle will be so short 

 that the twitching movements of the pale muscles produce little 

 or 110 discontinuity of movement, or tremor." 



On account both of its histological and physiological pro- 

 perties, cardiac muscle naturally falls under the same category as 

 the sluggish, sarcoplasmic, striated skeletal muscles. In corre- 

 spondence with its sluggish twitch and prolonged duration, we 

 might naturally expect to find it peculiarly adapted to steady, 

 complete tetanus. Yet the contrary results from experiment, and 

 in this respect, as in many others, cardiac muscle takes up a char- 

 acteristic attitude. Summation experiments are the more readily 

 carried out on the heart, since its spontaneous rhythmical con- 

 tractions, which are undoubtedly valid in a physiological sense 

 as single twitches, may be employed in a slow series of beats 

 (frog's heart) to investigate the action of a new artificial stimulus 

 (induction shock) in different phases of contraction and relaxation. 

 In these experiments Marey (17) found that cardiac muscle in 

 certain phases of its activity was variably sensitive to excitation 

 by a single induction shock, while during one period it is not at all 

 excitable (refractory). The ventricle, and all other sections of the 

 heart, are unresponsive to moderate stimuli during the entire 

 systole of the parts in question, while in the diastolic period, as 

 well as in the pause between each stimulus, an extra contraction is 

 yielded. With stronger excitation this " refractory period " is 

 more and more abbreviated, and very strong stimuli seem finally 

 to produce an excitatory effect in every sphere of cardiac activity 

 (Marey, Tigerstedt, Loven, etc.). This remarkable property of 

 all cardiac muscle will partially explain the characteristic reaction 

 of the heart during a rapid succession of (tetanising) stimuli ; 

 for it is evident that in consequence of this peculiarity a con- 

 stant, or rapidly repeated, excitation must fail to produce any 



K 



