558 EMBRYOLOGY. 



left compartments, and the truncus arteriosus into arteria pul- 

 monalis and aorta, and how in this way the heart attains its definite 

 form. 



The partitions arise independently in each of the three divisions 

 of the heart mentioned. 



Let us first take into consideration the atrium, which is for a 

 time the largest and most capacious region of the cardiac sac 

 s(fig. 308). In Man a separation into left and right halves (Iv and rv) 

 is observable even in the fourth week, since there is then formed 



on its hinder [dorsal] and 

 upper wall a perpendicular 

 projection inward, the first 

 trace of the atrial partition 

 (vs) or septum atriorum. 



The halves are even now 

 distinguished by the fact 

 that they receive different 

 venous trunks. The vitel- 

 line and umbilical veins, 

 as well as the Cuvierian 

 ducts to be discussed later, 

 empty their blood into the 

 right compartment, not 

 directly, however, and by 

 means of separate orifices, 

 but after they have united 

 with one another in the 

 vicinity of the heart to 

 form a large venous sinus 

 (sr) the sinus venosus or 

 s. reunions. This is imme- 



Fig. 308. Heart of a human embryo 10 mm. long, 

 neck measurement ; posterior [dorsal] half of the 

 heart, the front walk of which have been removed. 

 After His. 



t, Partition of the ventricle ; Ik, left, rk, right ven- 

 tricle ; ok, auricular canal ; Iv, left, rv, right 

 atrium ; sr, mouth of the sinus reunions ; vs, par- 

 tition of the atrium (atrial crescent, His ; septum 

 primuin, BORN) ; * Eustachian valve ; Ps septum 

 spurium. 



diately adjacent to the atrium and communicates with it by means 

 of a large opening in its posterior [dorsal] wall, which is flanked on 

 the right and on the left by a large venous valve (*). Only one 

 small vessel, which traverses the musculature of the heart obliquely, 

 opens, near the atrial partition, into the left compartment ; it is 

 the previously mentioned unpaired pulmonary vein, which is formed 

 immediately outside the atrium by the union of four branches, two 

 of which come from each of the two wings of the lung now being 

 established. 



In the further course of development the atrial partition grows 



