196 THE VASCULAR MECHANISM. 



escence, commence to beat, apparently spontaneously, and will continue so 

 beating for a long period of time. It may be said that in this case the dis- 

 tention of the cavity and the supply of blood or other fluid acts as a stim- 

 ulus ; but if so the stimulus is a continuous one, or at least not a rhythmic 

 one, and yet the beat is most regularly rhythmic. 



Then, again, the reluctance of the ventricle to execute spontaneous 

 rhythmic beats is to a certain extent peculiar to the frog. The ventricle of 

 the tortoise, for instance, the greater part of the substance of which is as 

 free from nerve cells as is that of the i'rog, will beat spontaneously when 

 isolated from the auricles with great ease and for a long time. Further, a 

 mere strip of this ventricular muscle tissue, if kept gently extended and 

 continually moistened with blood or other suitable fluid, will continue to 

 beat spontaneously with very great regularity for hours, or even days, 

 especially if the series be started by the preliminary application of induc- 

 tion-shocks rhythmically repeated. 



In connection with this question we may call attention to the fact that 

 the cardiac muscular fibre is not wholly like the skeletal muscular fibre ; in 

 many respects the contraction or beat of the former is in its very nature 

 different from the contraction of the latter ; the former cannot be considered, 

 like the latter, a mere instrument in the hands of the motor nerve fibre. 

 The features of the beat or contraction of cardiac muscle maybe studied on 

 the isolated and quiescent ventricle, or part of the ventricle, of the frog. 

 When such a ventricle is stimulated by a single stimulus, such as a single 

 induction-shock or a single touch with a blunt needle, a beat may or may 

 not result. If it follows it resembles, in all its general features at least, 

 a spontaneous beat. Between the application of the stimulus and the first 

 appearance of any contraction is a very long latent period, varying accord- 

 ing to circumstances, but in a vigorous fresh frog's ventricle being about 

 0.3 second. The beat itself lasts a variable but considerable time, rising 

 slowly to a maximum and declining slowly again. Of course, when the beat 

 is recorded by means of a light lever placed on the ventricle, what the 

 tracing shows is really the increase in the front-to-back diameter of the 

 ventricle during the beat that is to say, one of the results of the contrac- 

 tion of the cardiac fibres and gives, in an indirect manner only, the extent 

 of the contraction of the fibres themselves ; and the same is the case with 

 the other methods of recording the movements of the whole ventricle. We 

 may, however, study in a more direct way the contraction of a few fibres 

 by taking a slip of the ventricle (and for this purpose the tortoise is prefer- 

 able to the frog) and suspending it to a lever after the fashion of a muscle- 

 nerve preparation. We then get a curve of contraction, characterized 

 by a long latent period, a slow long-continued rise, and a slow long-con- 

 tinued fall a contraction, in fact, more like that of a plain muscular fibre 

 than of a skeletal muscular fibre. In the tortoise the contraction is partic- 

 ularly long, the contraction of even the skeletal muscles being long in that 

 animal ; it is less long, but still long, in the frog ; shorter still, but yet long 

 as compared with the skeletal muscles, in the mammal. 



The beat of the ventricle, then, is a single but relatively slow, prolonged 

 contraction wave sweeping over the peculiar cardiac muscle-cell, passing 

 through the cement substance from cell to cell along the fibre, from fibre to 

 fibre along the bundle, and from bundle to bundle over the labyrinth of the 

 ventricular walls. 



Like the case of the skeletal muscle, this single contraction is accom- 

 panied by an electric change, a current of action. The intact ventricle at 

 rest is, as we have already said ( 66), isoelectric, but each part just as it is 

 about to enter into a state of contraction becomes negative toward the rest. 



