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MB. C. H. O DONOGHUE ON THE 



backward growth of two septa. One, the septum aortico- 

 pulnionale, arises from the edge of the pulmonary artery, and as 

 it grows it divides the truneus cavity into two tubes, an aortic and 

 a pulmonary. The other, the septum aorticum, arises between 

 the two aortic arches, and so subdivides the aortic cavity of 

 the truneus into two, a right and a left. Of the three tubes 

 formed in this way, one lies to the left, ventrally, and leads to the 

 pulmonary arch ; another lies to the right, ventrally, and leads 

 to the left aortic arch ; and the third lies dorsally and leads to 

 the right aortic arch. 



Text-fig. 86. 



Ventral view of the heart and adjoining vessels. 



Az. Azygos vein. C.A. Left common carotid artery. L.A. Left auricle. 

 L.C.A. Left coronary artery. L.J. Left common jugular vein. L.S. Left systemic 

 arch. P.A. Pulmonary artery. P.C. Primary carotid. P.V. Pulmonary vein. 

 P.V.C. Post-caval vein. R.A. Right auricle. R.C.A. Right coronary artery. 

 R.J. Right common jugular vein. R.S. Right systemic arch. Th.A. Thyroid 

 artery. V. Ventricle. V.A. Vertebral artery. 



Lastly, the sinus venosus also assumes its definitive form. 

 After the disappearance of the umbilical and omphalo-mesenteric 

 veins, it has opening into it, on the right, the right ductus 

 Ouvieri and the post-caval, while on the left it has the left ductus 

 Ouvieri. Thus we have practically the adult condition of the 



