1034 



THE VASCULAE SYSTEM. 



The perforations eventually blend together to form the secondary foramen ovale of the inter- 

 atrial septum. As soon as the septum primum is completed the primitive atrium is divided into 

 the permanent right and left atria, each of which communicates through the secondary foramen 



ovale, with the atrium of the 



Left part of atrio-ventricular orifice Truncus arteriosus 



Septum primum fusing with 



superior atrio-ventricular cushion 



Wall of left ' ' ' 



atrium 



Left 



posterior 



cardinal 



veins opened 



opposite side, and with the 

 corresponding portion of the 

 ventricle through an atrio- 

 ventricular aperture which is 

 completely separate from its 

 fellow of the opposite side. 



As the ventral border of 

 the septum primum fuses with 

 the endocardial cushions of the 

 atrio - ventricular canal, the 

 septum secundum appears, im- 

 mediately to the right of the 

 septum primum. It grows 

 from the dorsal and cranial 

 (superior in adult) walls of the 

 atrium. As the septum secun- 

 dum develops, the right cornu 

 of the sinus venosus is ab- 

 sorbed into the atrium, the 

 left venous valve fuses with 

 the septum primum or dis- 

 appears, and the angle between 

 the right horn and the inter- 

 FIG. 831. VIEW OF THE INTERIORS OF THE RIGHT AND LEFT ATRIA OF mediate part of the sinus ap- 

 A HUMA'N EMBRYO 5'5 MM. LONG. (Edinburgh University collection, pears in the caudal part of the 

 Modelled by C. C. Wang.) dorsal wall of the right atrium. 



As the septum secundum in- 

 creases in size its cephalic part grows first ventrally and then caudally and lastly dorsally ; conse- 

 quently the free border soon becomes concave and the concavity of its margin is directed dorsally. 

 Both extremities of the free margin of the septum secundum fuse with the right lateral surface 

 of the septum primum, and the more ventrally situated part of the border, growing along the 



Wall of right 

 atrium 



Septum primui 

 fusing with 

 inferior atrio- 

 ventricular 

 cushion 



Septum 

 " primum 



Left venou 

 " valve 



Right" 

 - venous 



valve 



Inferior atrio- 

 ventricular cushion 



Right posterior cardinal veins opened 



Right duct of Cuvier opening 

 into right horn of sinus venosus 



7th intersegmental artery Dorsal aorta 



Vertebral artery 



Basilar artery 



Posterior 

 / cerebral artery 



Aorta 



1st cephalic 

 I aortic arch 

 2nd cephalic aortic arch 

 3rd cephalic aortic arch 

 4th cephalic aortic arch 



Superior vena 



caval blood- 



stream 



6th cephalic aortic arch 



Septum dividing the bulbus cordis 



Internal carotid artery 



Interventricular septum 



Foramen ovale 



Inferior vena 

 caval blood- _ 

 stream 



Middle 

 'cerebral artei 



Anterior cerebral 

 artery 



Intera trial 

 septn m 



FIG. 832. DEVELOPMENT OF THE HEART AND THE MAIN ARTERIES. 

 Diagram of the heart, showing the formation of its septa, and of the cephalic portion of the arterial system. 



septum primum, fuses with the angle between the right horn and the intermediate part of the 

 sinus venosus forming with it the lower part of the limbus fossae oval is of the fully developed heart. 

 The remainder of the limbus is formed by the thickened free margin of the septum secundum. 

 As soon as the septum secundum passes beyond the level of the foramen ovale that portion 



