THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 565 



is gradually merged into the wall of the atrium. In consequence of 

 this the great venous trunks, which originally emptied their blood 

 into it and which have meanwhile been converted into the superior 

 and inferior venae cavse and into the sinus coronarius (the details of 

 which are given in section d), empty directly into the right half of 

 the atrium, and here gradually separate farther and farther from 

 one another. Of the two valves which surround, as was previously 

 stated, the mouth of the sinus venosus, the left becomes rudimentary 

 (figs. 308, 309) ; the right (*), on the contrary, persists at the mouth 

 of the inferior vena cava and of the sinus coronarius, and is divided, 

 corresponding to these, into a larger and a smaller portion, of which 

 the former becomes the valvula Eustachii, the latter the valvula 

 Thebesii. 



The four pulmonary veins are united for a time into a common 

 short trunk, which empties into the left half of the atrium. Sub- 

 sequently the common terminal portion becomes greatly enlarged 

 and merged with the wall of the heart, in the same way as the sinus 

 venosus does. In consequence the four pulmonary veins then open 

 separately and directly into the atrium. 



The foramen ovale, the formation of which was previously 

 described, maintains a broad communication between the two sides 

 of the atrium during the entire embryonic life. It is bounded 

 behind and below by the atrial partition, a connective-tissue mem- 

 brane that subsequently receives the name of valvula foraminis 

 ovalis (fig. 309 si). Also from above and in front there is formed a 

 sharp limitation, since a muscular ridge projects inward from the 

 atrial partition, the anterior atrial crescent or the limbus Vieussenii 

 (?;s). Even in the third month all of these parts are distinctly 

 developed ; the valvula foraminis ovalis already reaches nearly to 

 the thickened margin of the anterior muscular crescent, but is 

 deflected obliquely into the left half of the atrium, so that a broad 

 fissure remains open and permits the blood of the inferior vena cava 

 to enter into the left part of the atrium. After birth the margins 

 of the anterior and posterior folds come into contact, and, with 

 occasional exceptions, fuse completely. The posterior fold furnishes 

 the membranous partition of the foramen ovale ; the anterior, with 

 its thickened muscular margin, produces above and in front the 

 limbus Vieussenii. With this the heart has attained its permanent 

 structure. 



While the cardiac sac undergoes these complicated differentiations, 

 it changes its position in the body of the embryo and acquires at an 



