950 



PHYSIOLOGY 



other hand the phenomena are easily explained on the assumption that the 

 whole of the musculature of the heart acts in many respects as a single 

 muscle fibre, along which an excitatory process may be propagated in any 

 direction. But in the adult mammalian heart, on superficial dissection, 

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

 cartilaginous ring surrounding the auriculo-ventricular junction, leaving 

 apparently no muscular continuity between the two cavities. On this 

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

 tion from auricles to ventricles must 

 occur by means of nerve fibres, and 

 it was only with the ^di son vp.ry hy 

 His of a distinct band of modified 

 muscle 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. 430). The musculature of these 

 chambers is continuous throughout. 

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

 anatomical relations of the different 

 cavities have become considerably 

 modified in the course of develop- 

 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 terminalis 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 and to the auricular ring surrounding the auricu- 

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

 as the intraventricular 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 which are known 



FIG. 430. A generalised type of 



vertebrate heart. (KEITH.) 

 a, sinus venosus ; &, auricular canal ; 

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

 /, aorta ; 1-1, sino-auricular junction and 

 venous valves ; 2-2, canalo-auricular junc- 

 tion ; 3-3, annular part of auricle ; 4-4, in- 

 vaginated part of auricle ; 5, bulbo-ventri- 

 cular junction. 



