1058 PHYSIOLOGY 



ventricle, the sinus muscle fibre representing the most primitive condition. 

 There is complete muscular continuity between all the cavities of the heart. 

 The circular ring of muscle at the junction of sinus with auricles and of auricles 

 with ventricles presents only slight traces of cross-striation. 



The heart is well supplied with nerve fibres and ganglion-cells* The two vagi 

 enter the sinus venosus and branch just under the pericardium. Here they 

 become connected with a collection of nerve-cells, spoken of 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-ventri- 

 cular groove they enter two collections 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. 424) the two auricles are bound together by a flat band 

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



B ' A - R.V.C.S /^ /AuRiCLt 



SINUS 



VENTRICLE 



"^ N / KSUHfA'Mjl 



V.C.I 



FIG. 423. Diagram of frog's heart. (After p ia 424. 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.O.S and R.V.C.S, left and right su- s, nerve-trunk with fibres con- 



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



IT. A, truncus arteriosus. ganglia ; COR. V, coronary vein. 



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

 vagi 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 consider- 

 ably 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's ligature), 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 



