THE CAUSATION OF THE HEART BEAT 983 



the sinus venosus and branch just under the pericardium. Here they become con- 

 nected with a collection of nerve cells, known as Remak's ganglion. From the sinus 

 the two vagi, now called septal nerves, pass down in the inter auricular septum, one in 

 front and the other behind. Near the auriculo-ventricular groove they enter two collec- 

 tions of ganglion cells, called Bidder's ganglia. From these ganglia non-medullated 

 fibres are distributed to surrounding parts of the auricle and to the whole of the ventricle. 

 In the upper third of the ventricle occur scattered ganglion cells attached to the nerve 

 fibres. These are quite absent in the lower half or two-thirds. 



In the tortoise (Fig. 431) the two auricles are bound together by a flat band of 

 tissue, which serves also to connect the sinus with the ventricle. The septum between 



AURICLE 



V.C.I 



VENTRICLE 



WH 



COR.V. 



FIG. 430. Diagram of frog's heart. (After FIG. 431. Tortoise's heart (after 



CYON.) GASKELL) as it appears when sus- 



v, ventricle; R.A, L.A, right and left auricles pended for registering the auricu- 



(atrium)- s.v, sinus venosus; P.v, pulmonary lar and ventricular contractions. 



veins; L.v.c.s and R.V.C.S, left and right su- N, nerve-trunk with fibres con- 



perior vena cava; v.c.i, vena cava inferior: necting Remak's and Bidder's 



TT.A, truncus arteriosus. ganglia ; COR. v, coronary vein. 



the auricles arises from the central line of this junction wall. The two vagus nerves 

 pass into a large accumulation of ganglion cells in the sinus, and thence along the basal 

 wall to the auriculo-ventricular groove, lying just under the pericardium. In the groove 

 they pass into a collection of ganglion cells, whence fibres are given off to both auricles 

 and ventricle. As they leave the sinus, a branch is always given off by the right nerve 

 to accompany the coronary vein, which conveys blood from the ventricular wall to the 

 sinus. Thus the nerves of the tortoise's heart are altogether more accessible than those 

 of the frog's heart. In other points the tortoise's heart is similar to the frog's heart, 

 though considerably larger. 



THE AUTOMATIC CONTRACTION OF THE FROG'S HEART 



The frog's heart in the body, or when removed from the body intact, 

 beats regularly, the contraction starting in the sinus, then travelling to 

 auricles, ventricle, and bulbus. If however the heart be removed by 

 cutting it across the sino-auricular junction, or if the auricles be functionally 

 separated from the sinus by a ligature round this junction (Stannius' liga- 

 ture), the auricles and ventricle stop in an uncontracted condition (diastole), 

 while the sinus goes on beating regularly. After the lapse of a period varying 

 from five minutes to half an hour, the detached part of the heart begins to 

 beat, at first slowly and then more rapidly, but never attaining the rate of the 

 sinus. The auricles beat first, and then the ventricle. If now the ventricle 

 be cut away by an incision in the auriculo-ventricular groove from the auricles, 

 the latter go on beating; while the former, after a few beats due to the 

 excitation of the incision, stops beating, and only after a considerable time 

 may begin again to contract very slowly. 



On the other hand, a ventricle-apex preparation (that is to say, the lower 



