THE VASCULAR MECHANISM. 197 



Hence, when the electrodes of a galvanometer are placed on two points, 

 A, B, of the surface of the ventricle, a diphasic variation of the galvanom- 

 eter needle is seen just as a beat, natural or excited, is about to occur. Sup- 

 posing that the wave of contraction reaches A first, this will become nega- 

 tive toward the rest of the ventricle, including B, but when the wave some 

 time afterward reaches B, B will become negative toward the rest of the 

 ventricle, including A. Compare 67. 



The beat of the auricles, that of the sinus venosus, and that of thebulbus 

 arteriosus are similar in their main features to that of the ventricle, so that 

 the whole beat may be considered to be a wave of contraction sweeping 

 through the heart from sinus to bulbus ; but the arrangement of fibres is 

 such that this beat is cut up into sections in such a way that the sinus, the 

 auricles, the ventricle, and thebulbus have each a beat, so to speak, to them- 

 selves. In a normal state of things these several parts of the whole beat 

 follow each other in the sequence we have described, but under abnormal 

 conditions the sequence may be reversed, or one section may beat while the 

 others are at rest, or the several sections may beat out of time with each 

 other. 



So far the description of the contraction which is the foundation of the 

 beat differs from that of a skeletal muscle in degree only ; but now comes an 

 important difference. When we stimulate a skeletal muscle with a strong 

 stimulus we get a large contraction ; when we apply a weak stimulus we get 

 a small contraction ; within certain limits (see 77) the contraction is pro- 

 portional to the stimulus. This is not the case with the quiescent ventricle 

 or heart. When we apply a strong induction-shock we get a beat of a cer- 

 tain strength ; if we now apply a weak shock, we get either no beat at all or 

 quite as strong a beat as with the stronger stimulus. That is to say, the 

 magnitude of the beat depends on the condition of the ventricle (or heart), 

 and not on the magnitude of the stimulus. If the stimulus can stir the ven- 

 tricle up to beat at all, the beat is the best which the ventricle at the time 

 can accomplish ; the stimulus either produces its maximum effect or none at 

 all. It would seem as if the stimulus does not produce a contraction in the 

 same way that it does when it is brought to bear on a skeletal muscle, but 

 rather stirs up the heart in such a way as to enable it to execute a spon- 

 taneous beat, which, without the extra stimulus, it could not bring about. 

 And this 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 obvi- 

 ously produced by the stimulus. But if it be thrown in just as a contraction 

 is beginning, no premature beat follows ; the ventricle does not seem to feel 

 the stimulus at all. There is a period during which the ventricle is insen- 

 sible to stimuli, and that however strong ; this period is called the " refrac- 

 tory " 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 show 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, 



