THE CAUSATION OF THE HEART BEAT 



993 



the muscle fibres both of auricles and ventricles are seen to arise from a fibro- 

 cartilaginous ring surrounding the auriculo-ventricular junction, lea vino- 

 apparent ly no muscular continuity between the two cavities. On this 

 account it was thought for many years that the propagation of the contrac- 

 ion from auricles to ventricles must 



ur by means of nerve fibres, and 



was only with the discovery by 

 of a distinct band of modified 

 uscle fibres, passing from the auri- 

 cles to the ventricles, the * auriculo- 

 ventricular bundle/ that an anato- 

 mical basis was furnished for the 

 physiological behaviour of the heart. 

 The heart is developed from a mus- 

 cular tube in which at the beginning 

 we must assume muscular continuity 

 throughout. The primitive vertebrate 

 heart is formed by a modification of 

 this muscular tube. In this heart, as 

 Keith has shown, we may distinguish 

 five chambers, namely, the sinus 

 venosus, the auricular canal, the 

 auricle, the ventricle and the bulbus 

 (Fig. 439). The musculature of these 

 chambers is continuous throughout, 



In the adult heart, e.g. of man, the 



i ! ,. ,1 j-ev 



anatomical relations ot the ditterent 



' 



0> 



venosus . 6> auric ular canal; 



c, auricle; d, ventricle; e, bulbus cqrdis; 

 f> aorta; 1-1, sino-auricular junction and 

 venous valves ; 2-2, canalo^uricular junc- 

 cavities have become considerably tion; 3-3 ' annular pa rt of auricle; 4-4, 



,.--,. ,, * i , invagmated part of auricle; 5, bulbo- 



modified in the course of develop- ventricular junction. 



ment. The sinus venosus, i. e. the 



part where in the lower vertebrates the contraction wave takes its 

 origin, is now represented merely by the termination of the superior 

 vena cava and of the coronary sinus in the right auricle. These two 

 veins are derived from the right and left ducts of Cuvier in the embryo. 

 The sinus venosus is also represented by a small amount of tissue under- 

 lying the tcenia terminal is of the right auricle, as well as by the remains 

 of the Eustachian and venous valves. The auricular canal gives rise 

 to the auricular septum ai^. to the auricular ring surrounding the auricu- 

 lar-ventricular orifice, and in some hearts it is prolonged into the ventricle 

 as the intra ventricular or invaginated part of the auricular canal. In the 

 adult heart two accumulations of more primitive tissue are found in the 

 region corresponding to the sinus venosus of the embryo, and these are known 

 as the sino-aiirieular node and the auriculo-ventricular node. The sino- 

 auricular node (Fig. 440) lies in the groove between the superior vena cava 

 and the right auricle. The auricular- ventricular node lies at the base of 

 the auricular septum on the right side, below and to the right of the opening 

 63 



