DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM 761 



the embryo and the umbilical veins from the placenta subsequently open into it (Fig. 545). The 

 sinus is at first placed transversely, and opens by a median aperture into the common auricle. 

 Soon, however, it assumes an oblique position, and becomes crescentic in form; its right half 

 or horn increases more rapidly than the left, while the opening into the auricle now communi- 

 cates with the right portion of the auricular cavity. The right horn ultimately becomes incor- 

 porated with and forms a part of the right auricle, the line of union between it and the auricle 

 proper being indicated in the interior of the adult auricle by a vertical crest (crista terminalis of 

 His). The left horn, which ultimately receives only the left duct of Cuvier, persists as the 



Right auricl 

 Bulbus cordis 



-ILeft auricle 



Auricular 

 canal 



Ventricle 



FIG. 537. Heart showing expansion of auricles. (Drawn from Ecker-Zeigler model.) 



coronary sinus (Fig. 494). The vitelline and umbilical veins are soon replaced by a single 

 vessel, the inferior vena cava, and the three veins (inferior vena cava and right and left Cuvierian 

 ducts) open into the dorsal aspect of the auricle by a common slit-like aperture (Fig. 542). 

 The upper part of this aperture represents the opening of the permanent superior vena cava, 

 the lower that of the inferior vena cava, and the intermediate part the orifice of the coronary 

 sinus. The slit-like aperture lies obliquely, and is guarded by two valves, the right and left 

 venous valves, which unite with each other above the opening and are continuous with a fold 

 named the septum spurium. The left venous valve practically disappears, while the right is 

 subsequently divided to form the Eustachian and Thebesian valves. At the lower extremity 



Aortic bulb. 

 Left auricle. 

 Left ventricle. 



Bight auricle. 



.Superior vena cava. 

 ' Septum transversum. 



-Umbilical vein. 



Vitelline or Omphalo-mesenteric vein. 

 FIG. 538. Heart of human embryo, 4.2 mm. long, seen from behind. (His.) 



of the slit is a triangular thickening, the spina vestibuli of His, which partly closes the aperture 

 between the two auricles, and, according to His, takes a part in the formation of both the inter- 

 auricular and interventricular septa. 



The auricular canal is at first a short straight tube connecting the auricular with the ven- 

 tricular portion of the heart, but its growth is relatively slow, and it becomes overlapped by the 

 auricles and ventricles so that its position on the surface of the heart is indicated only by an 

 annular constriction (Fig. 537). Its lumen is reduced to a transverse slit, and two thickenings 

 appear, one on its dorsal and another on its ventral wall. These thickenings, or endocardial 

 cushions (Fig. 542), as they are termed, project into the canal, and, meeting in the middle line, 



