1190 EMBRYOLOGY. 



auricle, and is formed by the union of three pairs of veins, viz., (1) the veins or 

 ducts of Cuvier from the body of the embryo ; (2) the omphalo-mesenteric veins 

 from the yolk-sac ; (3) the umbilical veins from the placenta (Fig. 747). 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 its right 

 half or horn becomes larger than the left, while the opening into the auricle is 

 found to be in the right portion of the auricular cavity. The right horn ultimately 

 becomes incorporated 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, the crista terminalis of His. The left horn, 

 which ultimately receives only the left duct of Cuvier, persists as the coronary 

 sinus (Fig. 753). The omphalo-mesenteric 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. The upper part of this aperture represents the opening 



Aortic bulb. 



Left auricle. V^ ^^ 

 Left ventricle.-, \ /^ Ki 9 ht auricle. 



.Superior vena cava. 

 Septum transversum. 



-Umbilical vein. 



Vitellinc or Omphalo-me.tenteric vein. 

 FIG. 747. Heart of a human embryo 4.2 mm. in length, seen from behind. (His.) 



of the permanent superior vena cava, the lower part that of the inferior vena cava, 

 and the intermediate part the orifice of the coronary sinus. The slit-like aperture 

 lies obliquely, and is bordered on its mesial and lateral aspects by a fold of endo- 

 cardium. The mesial part of the fold disappears, while from the lateral part the 

 Eustachian arid Thebesian valves are developed. At the lower extremity of the 

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

 aperture between the two auricles, and which, according to His, takes a part in the 

 formation of both the interauricular and interventricular septum. 



The common auricle becomes gradually subdivided into right and left auricles 

 by a septum, the septum superius, which grows from its dorsal and upper wall so 

 that the two auricles communicate with each other only below the margin of this 

 septum. This communication (ostium primum of Born) does not, however, represent 

 the future foramen ovale, for the septum grows downward and blends with the 

 partition which comes to subdivide the auricular canal. The foramen ovale 

 (ostium secundum of Born) results from a perforation of the upper part of the 

 septum superius. 



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

 the ventricular portion of the heart, but it becomes overlapped by the growing 

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

 only by an annular constriction (Fig. 746). Its lumen is reduced to a trans- 

 verse slit, and a thickening appears on its dorsal and ventral walls. These thick- 

 enings, or endocardial cushions as they are termed, project into the canal, and, 

 meeting in the middle line, divide the canal into two channels, the future right 

 and left auriculo-ventricular orifices. 



The common ventricle becomes divided by a septum, the septum inferius, which 



