568 THE HUMAN EMBRYO. 



than before, opposite the hyoidean and first branchial arches 

 (Fig. 215) . The structure of the heart is the same as in the earlier 

 stages, except that the muscular elements have increased consider- 

 ably. The wide space between the muscular and the endothelial 

 walls is well shown in Fig. 215, as is also the fibrous network 

 connecting the two walls. 



In describing the further development of the heart it will be 

 convenient to take the several divisions one by one. 



The sinus venosus. The blood is returned to the heart by 

 three main veins on each side : the Cuvierian vein (Fig. 243, 

 VD), from the body of the embryo ; the vitelline vein, from the 

 yolk-sac ; and the allantoic vein, from the placenta. These three 

 pairs of veins form by their union a single large vessel, the 

 sinus venosus, which runs transversely across the body, imme- 

 diately in front of the liver, and opens through a median aper- 

 ture into the auricular portion of the heart. 



The sinus venosus is at first situated behind the diaphragm, 

 but during the fourth week it gradually extends over, and in 

 front of this, and so comes to lie in the pericardial cavity, imme- 

 diately behind the auricle (Fig. 243). 



Towards the end of the fourth week the sinus venosus 

 becomes placed somewhat obliquely, in place of transversely, 

 across the body ; at the same time its right side becomes larger 

 than the left, and the opening into the auricular cavity, which was 

 at first median, shifts so as to lead distinctly into the right side 

 of the auricle (Fig. 244, RS). During the fifth week, the open- 

 ing from the sinus venosus into the auricle widens out very con- 

 siderably, so that the sinus becomes part of the auricle itself, 

 and ceases to exist as a separate cavity. The left horn of the 

 sinus venosus, which now only receives the left Cuvierian vein, 

 retains its independence more completely, and persists as the 

 coronary sinus. 



The auricles. The auricular chamber is at first single, but 

 towards the end of the fourth week it becomes imperfectly divided 

 into the right and left auricles (Fig. 244). The division is 

 indicated externally by a slight constriction, and more markedly 

 by the outgrowth of the auricular appendices, which very early 

 show characteristic crenations along their margins. 



