AO'RTA, from a Greek word, hop-rh. The aorta is the great verso from which all the arteries of the body which carry red blood derive their origin. It arises from the upper and back part of the left ventricle of the heart. Its origin is directly opposite the lower margin of the cartilage of the third rib on the right side of the chest. From this point it ascends behind the pulmonary artery, still inclining a little to the right side of the chest. It continues to ascend as far as the top of the second vertebra of the back. All this part of the vessel is called the Aorta Ascendens. When it reaches as high as the lower margin of the first rib, it bends obliquely backwards towards the body of the third vertebra of the back. This part of the vessel is called the Curvature or the Transverse Arch of the Aorta. From the third vertebra of the back, where its arch terminates, it proceeds in a straight course downwards through the chest, immediately in front of the spinal column, and towards the left side of it. Through an
opening formed for it in the diaphragm it passes from the chest into the abdomen. All this part of the vessel, namely that extending between the termination of the arch and the diaphragm is denominated the Descending or the Straight Portion of the Thoracic Aorta. Having passed through the diaphragm iuto the abdomen, it is called the Abdominal Aorta. It continues to descend along the front of the spine a little obliquely, until it reaches the fourth vertebra of the loins : here it divides into two branches of equal size, and may be said to terminate, for it now loses the name of aorta ; the two great branches into which it divides being denominated the Common Iliac Arteries. [II EanT.]