THE HEAET. 801 



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 truncus arteriosus, into the aortic arches. 



The i nternal division of the heart, into right and left sides, 

 is effected b y three septa or partitions, whic h appear within the 

 cavit y of the heart , and which ari se perfectly independe ntly" of 

 one another : (i) the interauricular septu m, which divides the 

 auricular chamber int o th e rig ht and left auricle s; (ii) the 

 iiiter Yeiitricular~septum , which divides t he ventricular cha mber 

 into the right and left ventric les ; (iii) t he septum of the trun cus 

 arteriosus, whi ch divides the trnncus a,rtp,rioH us. or terminal 

 chamber o f the hear t, into right and left halv es. Of these septa, 

 the tirst 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 intera uricular sept um 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 



