THE HEAET. 301 



During the third day (Fig. 113), the heart increases con- 

 siderably in size ; the S-like twisting becomes still more pro- 

 nounced than before ; and constrictions appear, separating the 

 several chambers of the heart from one another. 



On the fourth day, the auricular portion of the heart becomes 

 widened laterally, and marked off by a sharp constriction from 

 the ventricular portion, which, in its turn, is separated by a 

 distinct though less pronounced constriction from the truncus 

 arteriosus. 



The most important event, however, that happens during 

 the fourth day, so far as the heart is concerned, is the first 

 appearance of the partitions by which the right and left sides of 

 the heart become separated from each other. Up to the fourth 

 day the heart is a single and continuous, though twisted tube, 

 without any division whatever into right and left sides. The 

 blood enters at the posterior or venous end of the heart, and 

 passing through the several cavities in succession passes out in 

 front, through the truiicus arteriosus, into the aortic arches. 



The internal division of the heart, into right and left sides, 

 is effected by three septa or partitions, which appear within the 

 cavity of the heart, and which arise perfectly independently of 

 one another : (i) the interauricular septum, which divides the 

 auricular chamber into the right and left auricles; (ii) the 

 interventricular septum, which divides the ventricular chamber 

 into the right and left ventricles ; (iii) the septum of the truncus 

 arteriosus, which divides the truncus arteriosus, or terminal 

 chamber of the heart, into right and left halves. Of these septa, 

 the first two commence to form on the fourth day ; the third, or 

 septum of the truncus arteriosus, does not arise until the fifth 

 day. 



Concerning the relative times of appearance of the inter- 

 auricular and interventricular septa, there is some discrepancy 

 in the published accounts. It is commonly stated that the 

 interventricular septum develops the earlier of the two, but 

 according to Masius it is the interauricular septum which is 

 the first to be formed. 



The interauricular septum appears, during the fourth day, as 

 a septum projecting into the auricular chamber from its anterior 

 and dorsal wall; it lies between the apertures of the sinus 



