38 THE HEART 



auricular junction that is, where the superior vena cava joins the 

 tsenia terminalis of the right auricle (beneath a, Fig. 4). This corre- 

 sponds in position to the right venous valve of the sinus venosus 

 of the primitive heart. The remnant has been termed the " sino- 

 auricular node." .It is interesting because it is in close muscular 

 connection (1) with the outer wall of the auricle ; (2) with the 

 interauricular septum. In the latter fibres pass from the node 

 down the septum to another remnant of primitive fibres at the 

 base of the septum known as the auriculo-ventricular or A-V 

 node. To this we shall refer again, but it is worthy of note that 

 the two nodes, identical in structure, and therefore probably in 

 function, are in muscular connection with each other. 



The sino-auricular node has a special blood supply, and the 

 nerves in the neighbourhood come into intimate relationship with 

 it (Keith and Flack). 



The Auricular Canal of the Human Heart. It will be seen 

 that in the simple form of vertebrate heart (Fig. 1) the auricular 

 canal consists of three parts : 



(1) A basal part opposite the auricle. 



(2) An annular part or " auricular ring " (3.3). 



(3) An invaginated or intraventricular part (4.4). 



The basal part is the ventral wall of the primitive cardiac tube. 

 The auricle (c) has developed from the dorsal wall alone, leaving 

 the ventral wall unspecialised. The basal wall is therefore con- 

 tinuous with 



(a) The sinus .venosus. 



(6) With the ostium of the auricle. 



(c) With the auricular ring. 



Does this continuity persist in the mammalian heart ? In this 

 heart the baSal wall has become profoundly modified owing to 

 the formation of an interauricular septum and a vestibule to the 

 left auricle. Both these structures have been developed from 

 the primitive basal wall. 



The " auricular ring " is that portion of the auricular canal 

 interposed between the auricle and the ventricle (3.3, Fig. 1), and 

 in these hearts is of comparatively appreciable dimensions. In 

 the mammalian heart, however, it is represented by a small 

 but nevertheless important remnant. This is submerged in the 

 auriculo-ventricular groove at the junction of the auricles and 

 ventricles. The shape of " the ring," however, has become 



