1070 PHYSIOLOGY 



The movement in the reverse direction at Q may either represent the 

 passing off of the auricular contraction, or, more probably, the beginning 

 of the ventricular contraction somewhere near the apex, e.g. in the 

 neighbourhood of the papillary muscles. The large wave R is certainly 

 due to contraction of the base of the ventricles, and the rapid descent 

 of the curve signifies the spread of the excitation wave over the whole 

 heart, involving the apex. The wave T is almost certainly due, as 

 the similar excursion in the electrocardiogram of the frog's ventricle, 



to the contraction of muscle fibres 

 surrounding the root of the aorta 

 and the pulmonary artery, corre- 

 sponding to the bulbus aortse in 

 the frog. 



The explanation of the order of 

 the different phases in the curve 

 of the electrical changes is to be 

 sought in the arrangement of the 

 muscular bundles or associated tis- 

 sues which are responsible for the 

 propagation of the wave through 

 the heart. In the primitive verte- 

 brate heart, as Keith has shown, 

 we may distinguish five chambers, 

 namely, the sinus venosus, the auri- 

 cular canal, the auricle, the ven- 

 tricle (Fig. 434) and the bulbus 



(Fig. 434). The musculatureof these 



chambers is continuous through- 

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

 man, the anatomical relations of 

 the different cavities have become 

 considerably modified in the 

 course of development. 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 underlying 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 sur- 

 rounding the auriculo- ventricular orifice, and in some hearts it is 

 prolonged into the ventricle as the intraventricular or invaginated 

 part of the auricular canal. This intraventricular part is not at first 



FIG. 434. A generalised type of verte- 

 brate heart. (KEITH.) 

 a, sinus venosus ; 6, auricular canal ; 

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

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

 venous valves; 2-2, canalo -auricular 

 junction ; 3-3, annular part of auricle ; 

 4-4, invaginated part of auricle ; 5, 

 bul bo -ventricular junction. 



