THE MECHANISM OF TRANSPORT 119 



the junction of the two great veins. The blood enters the heart, 

 and the wave of contraction begins here. 



Chamber (b) represents the original cardiac tube from which the 

 auricle has grown out dorsally. It is known as the " auricular canal," 

 and may be subdivided into three parts: (^l) the part of the cardiac 

 tube antecedent to, and opposite, the outgrowth of the auricle (2-2) 

 generally termed the " basal part "; (B) the part which .comes after 

 the outgrowth of the auricle and before the downgrowth of the ven- 

 tricle (3-3) the " auricular ring " ; (C) a part (4-4) which has become 

 invaginated into the ventricle. The ventricle is represented by (d), 

 while (e) at the arterial end of the cardiac tub3 represents the chamber 

 known as the " bulbus cordis." 



FIG. 28. RIGHT AURICLE SEEN FROM THE SIDE. (Keith and Flack.) 



, Superior vena cava; 6, appendix; S.-A., sinu-auricular node; c, vestibule of left 

 auricle ; /, union in sulcus terminalis of two branch arteries arising from right 

 coronary artery; g, another anastomosing branch from right coronary artery; 

 f, inferior vena cava; /, aorta. 



In such a heart the flow through the organ is directed by four 

 sets of valves: (1) Placed between the sinus and auricular canal (the 

 venous valves) ; (2) at the auricular ring at the entrance to the ven- 

 tricle ; (3) and (4) at either end of the bulbus cordis. 



The two auricles of the mammalian heart are formed by a fusion 

 of the musculature of three parts of the primitive vertebrate heart 

 the sinus, the auricular canal, and the primitive auricle. 



The two ventricles are developed side by side from the ventral 

 wall of the primitive tube, an infolding of the walls fusing to form 

 the inter ventricular septum. 



The bulbus cordis comes to be represented by the infundibular 

 part of the right ventricle. 



In the mammalian heart the sinus venosus, with the venous valves, 

 has almost disappeared. Its most important remnant is a small 

 mass of tissue at the junction of the superior vena cava and the auricle 

 <Fig. 28) the sinu-auricular node. 



In the human heart the sinu-auricular node is about the size of 



