DEVELOPMENT OF THE HEART. 



707 



Neural canal 



Aorta 



left half. The interauricular partition, however, is not complete, since an opening 

 appears in its upper part even before it has finished its downward growth and union 

 with the valvular septum. This opening enlarges and remains as \h& forame^i ovale 

 that persists until birth as a direct passage for the blood from the right into the left 

 auricle during the continuance of the foetal circulation (page 929). 



The subdivision of the ventricular chamber, which commences a little later 

 than that of the auricle, is accomplished chiefly by the formation of the veritricular 

 septum. The latter grows from the postero-inferior wall of the ventricle as a cres- 

 centic projection that continues inward, a thickening of the ventricular wall corre- 

 sponding in position with the external interventricular groove. The partition thus 

 formed extends towards the auriculo-ventricular opening, where it meets and fuses 

 with the septum ihtermedium, in this manner insuring the communication of the 

 right and left auricles with the corresponding ventricles through the intervening 

 respective portions of the valvular opening. 



The isolation of the two ventricles irom each other, however, is not at first com- 

 plete, owing to the ventricular partition being imperfect above and in front. This 

 deficiency is overcome by the down- 

 ward extension of the aortic septum Fig 

 within the bulbus arteriosus (as the 

 somewhat dilated lower end of the trun- 

 cus arteriosus is now appropriately 

 called) until it meets and fuses with the 

 ventricular partition, thus completing 

 the separation of the cardiac chambers 

 into a right and left heart. The part 

 of the ventricular partition contributed 

 by the aortic septum always remains 

 thin and constitutes the pars mcmbra7i- 

 acea of the adult organ. 



Coincidently with the foregoing 

 changes, the auricles undergo impor- 

 tant modifications in their relations with 

 the blood-vessels opening into them. 

 During the development of the auricles, 

 the oval sinus venosus, into which is 

 conveyed the blood returned by the vi- 

 telline, allantoic (umbilical) and body- 

 veins, elongates transversely into a 

 crescentic sac, the convexity of which is 

 in contact with the back of the auricles, 

 its opening into the latter having shifted 

 so that it is in relation with only the 

 right half of the auricular chamber. With the expanded body and right horn of 

 the venous crescent communicate the vessels that later are represented by the superior 

 and inferior venae cavse, while the elongated and smaller left horn receives the left 

 duct of Cuvier that becomes the coronary sinus. 



For a time the opening of the sinus venosus, or si7ius reuniens (His), into the 

 heart occupies the posterior wall of the right half of the auricle. It is guarded by 

 the venous valve, consisting of a right and left leaflet, that is prolonged forward 

 along the roof of the auricle as a crescentic ridge, the septum, spurium (Fig. 674, A). 

 With the continued appropriation of the venous sinus by the expanding auricle, the 

 single aperture of the sinus disappears as the sac becomes part of the auricular 

 chamber, thereupon the two venae cavae and the coronary sinus open directly into the 

 right auricle by an independent orifice. That of the superior cava lies in the upper 

 posterior part of the auricle, that of the inferior cava being lower and more lateral, 

 with the smaller orifice of the coronary sinus slightly below. The septum spurium, 

 the greater part of the left, and the upper part of the right segment of the arching 

 fold that originally surrounds the opening of the sinus venosus disappear during the 

 appropriation of the venous sac by the auricle. The lower part of the right leaflet, 



Digestive 

 tube 



-*- Meso- 

 a cardium 



Primitive 

 auricle 



Endocardial 

 layer 



Myocardial 

 layer 



Primitive ventricle 



Transverse section of early rabbit em- 

 bryo passmg through young heart, showing 

 venous segment behind and arterial in 

 front. X 75- 



