304 FEATURES OF CARDIAC CONTRACTION. [BOOK i. 



which it is capable at the moment. This feature of the heart 

 beat is further illustrated by the fact that when a ventricle is 

 beating rhythmically either spontaneously, or as the result of 

 rhythmic stimulation, the kind of effect produced by a new 

 stimulus thrown in will depend upon the exact phase of the 

 cycle of the beat at which it is thrown in. If it is thrown in just 

 as a relaxation is taking place, a beat follows prematurely, before 

 the next beat would naturally follow, this premature beat being 

 obviously produced by the stimulus. But if it be thrown in just 

 as a contraction is beginning, no premature beat follows : the ven- 

 tricle does not seem to feel the stimulus at all. There is a period 

 during which the ventricle is insensible to stimuli, arid that how- 

 ever strong ; this period is called the ' refractory ' period. (There 

 is it may be mentioned a similar refractory period in skeletal 

 muscle, but it is of exceedingly short duration.) From this it 

 results that, when a succession of stimuli repeated at a certain rate 

 are sent into the ventricle, the number of beats does not correspond 

 to the number of stimuli, some of the stimuli falling in refractory 

 periods are ineffective and produce no beat. Hence also it is 

 difficult if not impossible to produce a real tetanus of the ventricle, 

 to fuse a number of beats into one. And there are other facts 

 tending to shew that the contraction of a cardiac muscular fibre, 

 even when induced by artificial stimulation, is of a peculiar nature, 

 and that the analogy with the contraction of a skeletal muscular 

 fibre, induced by motor impulses reaching it along its nerve, does 

 not hold good. 



These and other considerations, taken together with the facts 

 already mentioned that portions of cardiac muscular tissue in 

 which no ganglionic cells can even with the best methods be dis- 

 covered, may in various animals be induced, either easily or with 

 difficulty, to execute rhythmic beats, which have all the appear- 

 ance of being spontaneous in nature, lead us to conclude that the 

 beat of the heart is not the result of rhythmic impulses proceeding 

 from the cells of the ganglia to passive .muscular fibres, but is 

 mainly the result of changes taking place in the muscular tissue 

 itself. And here we may call attention to the peculiar histological 

 features of cardiac muscular tissue ; though so far differentiated 

 as to be striated, its cellular constitution and its ' protoplasmic " 

 features, including the obscurity of the striation, shew that the 

 differentiation is incomplete. Now one attribute of undifferen- 

 tiated primordial protoplasm is the power of spontaneous move- 

 ment. 



The further questions, By virtue of what internal molecular 

 changes the cardiac tissue is thus endued with spontaneous 

 rhythmic activity ? why the several parts, sinus, auricle, and 

 ventricle, are arranged in descending potency, so that the 

 cardiac cycle beginning with the sinus follows the course it does ; 

 why the contraction wave beginning at the sinus is broken up 



