CHAP, iv.] THE VASCULAR MECHANISM. 243 



entire heart. If the incision be carried between the auricles and 

 ventricle, the former with the sinus beat regularly and forcibly 

 while the latter often exhibits no spontaneous beats at all, or if 

 these do appear they last for a short time only. Lastly if the 

 ventricle be cut across leaving the upper third attached to the 

 auricles, this beats regularly with the sinus and auricles, but 

 the detached lower two-thirds do not beat spontaneously at all. 



Now, while ganglia are abundant over the sinus, are numer- 

 ous over the auricles, and, as Bidder's ganglia, are present at the 

 auriculo-ventricular junction, no nerve cells are present in the 

 lower part of the ventricle. Hence the view has suggested itself 

 that the rhythmic spontaneous beating is due to impulses pro- 

 ceeding rhythmically from the nerve cells of the ganglia. But 

 serious objections may be urged against this view. Even in the 

 case of the frog, the lower part of the ventricle, the mere tip 

 almost, may under specially favourable circumstances beat in an 

 apparently spontaneous manner ; this occurs when it is tied 

 round the end of a perfusion cannula (Fig. 66), and fed with blood 

 or serum at a somewhat high pressure. And in the case of the 

 tortoise a mere strip of muscle, quite free from nerve cells, cut 

 out of the ventricle may be made to beat, in an apparently spon- 

 taneous manner, with great regularity for a considerable time. 



Without entering into any lengthy discussion concerning a 

 matter which is and has been much debated, we may say that 

 the cardiac muscular fibre differs in properties, as it does in struc- 

 ture, from the skeletal muscular fibre, that it is not to the same 

 extent as that, a mere instrument, so to speak, in the hands of a 

 motor nerve fibre, but has, itself, largely to do with originating 

 its own contraction. The muscular contraction, we may here 

 observe, of which the beat is a development, is not a tetanus, 

 but a somewhat long continued simple contraction. This may 

 be readily shewn to be the case on the slip of the tortoise ven- 

 tricle just referred to. Such a slip, when attached to a lever, and 

 stimulated with a single induction shock, gives what is obviously 

 a simple contraction, and a beat of the slip occurring naturally 

 has exactly the same features. And the electric change shewn 

 at any part of the heart during a beat natural or induced by 

 stimulation is that characteristic of a simple contraction. The 

 intact ventricle at rest is as we have already said ( 63) isoelec- 

 tric, but each part just as it is entering into a state of contraction 

 becomes negative towards the rest. 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 galvanometer needle 

 is seen when a beat, natural or excited, occurs. Supposing that 

 the wave of contraction reaches A first, this will become negative 

 towards the rest of the ventricle, including B, but when the wave 

 sometime afterwards reaches B, B will become negative towards 

 the rest of the ventricle, including A. Compare 64. 



But the contraction of cardiac muscle differs from that of a 



