The narrowness may be at its origin or at the isthmus aortas. In the former case there is usually defect of the septa as already explained. Stenosis of the aorta, even amounting to atresia, may have little effect on the circulation during foetal life, the systemic circulation being carried on by the pulmonary artery through the ductus arteriosus. But at birth, as there is a great increase in the amount of blood coming to the left auricle by the opening up of the pulmonary circulation, the left ventricle may have great difficulty in dealing with it. In cases of stenosis of the isthmus aortse the circulation is assisted by anastomosis between the subclavian artery and the thoracic and abdominal aorta. The narrowness of the aorta at the isthmus may give rise in later life to aneurysm above the seat of narrowing.