THE MEANING OF THE HEART-BEAT. 177 



The evidence, then, points absolutely to the conclusion that the 

 muscular tissue of the heart possesses the power of rhythmicity, i.e. 

 answers to a sufficient constant stimulus with a series of contractions, 

 and that certain parts are more rhythmical than others, so that in them 

 the rhythmical contraction takes place automatically, without the 

 necessity of any continuous external stimulus. 



Upon what does this difference of rhythmicity depend ? It is 

 acknowledged on all sides that such difference is real, and it has been 

 largely supposed that the difference is due to the presence or absence of 

 ganglion cells, because ganglion cells are found in the sinus, in the auriculo- 

 ventricular junction, and between sinus and ventricle in those parts, in 

 fact, where the spontaneous rhythm is most easily observed ; also, because 

 a rhythmical action is supposed to necessitate a central nervous system, 

 and finally because there is apparently nothing else to which to attribute 

 this difference of rhythmical power. 



But is there really nothing else peculiar to those parts where 

 rhythm is easiest ? 



If we consider its mode of development, we find that in all verte- 

 brates the heart arises from two longitudinal veins, or venous sinuses, 

 which coalesce to form the sub-intestinal vein and heart; the simple 

 tube so formed, with its circular muscular coat, becomes twisted on 

 itself, and parts of it develop bulgings, the commencement of the 

 auricles and ventricles, in consequence of which the simple circular 

 arrangement of muscle fibres becomes the complex arrangement which 

 is ultimately found. With this special growth of the parts of the 

 tube by which the heart cavities are formed, is correlated a more rapid 

 contraction of the specialised parts of the muscular walls, so as to 

 ensure a more rapid emptying of the separate cavities, and thereby 

 make the heart more efficient as a force-pump. The development 

 of this nearer approach to striated muscle is made at the expense of 

 the original rhythmical power, so that finally the muscular tissue of the 

 heart becomes differentiated into muscle of varying rhythmical power, 

 according to the amount of deviation from the original embryonic 

 rhythmical muscle. 



Those parts which remained least altered, both in the circular 

 arrangement of their fibres and in their physiological status, are natur- 

 ally the large veins, sinus, and junction of sinus and auricles ; then the 

 circularly arranged fibres of the auriculo-ventricular groove, i.e. the com- 

 pressed remains of that portion of the original tube which, known as the 

 canalis auricularis, separated the auricular from the ventricular bulgings ; 

 and, lastly, the conus arteriosus, which represents the final part of the 

 tube beyond the ventricular bulging. In addition, there is in the 

 tortoise a basal portion of the tube leading from sinus to ventricle, 

 from which the bulged part of the auricles springs ; this may be looked 

 upon as a portion of the original tube, which has been only very slightly 

 differentiated. 



In fact, as can be readily understood, if the auricular and ventricular 

 bulgings are special modifications of the muscular tissue, for the purpose 

 of forming rapidly contracting roomy chambers in other words, for the 

 purpose of forming a heart rather than a rhythmically contractile tube 

 then the very parts which were left unmodified would naturally 

 continue to carry the main intracardiac nerves with their accompanying 

 ganglion cells ; so that in the sinus, in the auricles, and in the auriculo- 

 VOL. n. 12 



