CHAP, iv.] THE VASCULAR MECHANISM. 285 



mere tip of the frog's ventricle, that is to say parts which are 

 admitted not to contain nerve cells, may, by special means, be 

 induced to carry on for a considerable time a rhythmic beat, which 

 in its main features is identical with the spontaneous beat of the 

 ventricle of the intact heart. If such a part of the frog's ventricle 

 be tied on to the end of a perfusion cannula (Fig. 52), the portion 

 of the ventricular cavity belonging to the part may be adequately 

 distended arid at the same time be 'fed' with a suitable fluid, 

 such as blood, made to flow through the cannula; it will then 

 be found that the portion of ventricle so treated will, after a 

 preliminary period of quiescence, commence to beat, appaivntlv 

 spontaneously, and will continue so beating for a long period of 

 time. It may be said that in this case the distension of the 

 cavity and the supply of blood or other fluid acts as a stimulus ; 

 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 spon- 

 taneous 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 frog, 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 bkod or other suitable fluid, will continue to beat 

 spontaneously witn very great regularity for hours or even days, 

 especially if the series be started by the preliminary application of 

 induction 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 may be 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, vary- 

 ing according to circumstances, but in a vigorous fresh frog's 

 ventricle being about '3 sec. 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 shews 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 contraction of the 



