THE MEANING OF THE HEART-BEAT. 



173 



From this experiment I concluded, on the assumption that the 

 motor ganglia in the sinus are the cause of the heart-beat, that they 

 send out discrete and not continuous stimuli to the muscular tissue of 

 the various cardiac chambers. The next question that arose in my 

 mind was, Is it really clear that the sinus ganglia do originate nerve 

 impulses to the cardiac muscle ? or is it not that, whereas all parts of the 



FIG. 102. Heart of toad. Suspension method. Clamp in auriculo-ven- 

 tricular groove. Ventricle above clamp, auricle and sinus below. Coil 

 round ventricle heated between the vertical lines. The ventricular 

 contractions diminish in force during the heating of the ventricle, 

 but no alteration whatever occurs in the rate of beat. 



muscular tissue of the heart possess the power of rhythmical contraction 

 to a greater or less degree, some parts possess that power to such an 

 extent that their contractions are automatic ? 



To answer this question it appeared to me very necessary to 

 investigate other hearts than that of the frog, for in the frog the nerves 

 and ganglion cells between 

 the sinus and ventricle are 

 situated in the septum of the 

 auricles, and are therefore not 

 easily exposed for the pur- 

 poses of experiment. On the 

 other hand, in the case of the 

 tortoise, the septum between 

 the auricles is free from nerve 

 trunks and nerve cells, and 

 all the nerves and cells lie 

 superficially in the flat base 

 which connects the two aur- 

 icles, and continues on from 

 the sinus to the ventricle. 1 



This flattened part or junc- 

 tion-wall between the two aur- 

 icles is represented in Fig. 103, J. 

 On each side of it is seen the Fl ' K>3.-Heart of tortoise Description of letters 



reticulated tissue of the auricle 



proper (A), and the manner in which the junction-wall connects the sinus (S) 

 and ventricle ( V) is also shown. This basal auricular wall exists also in fishes 

 (described by Mac William 2 as sinus-extension), in crocodiles, and in Chelonia 

 generally, and in all cases carries the nerve trunks and coronary veins wherever 



1 Gaskell, "On the Innervation of the Heart, with especial reference to the Heart of 

 the Tortoise," Journ. PhysioL, Cambridge and London, 1883, vol. iv. p. 43. 



2 "On the Structure and Rhythm of the Heart of Fishes, with especial reference to the 

 Heart of the Eel," Journ. Physio!., Cambridge and London, 1885, vol. vi. p. 192. 



