AORTA is the great arterial trunk which, rising from the left ventricle of the heart, sends its branches ramifying through the whole body. The A. in man is subdivided by anatomists into the arch, the thoracic A., and the abdominal A. The arch is a loop with the convexity directed upwards, forwards, and to the right side, reaching at its highest part to a level with the second piece of the breast-bone, and then descending to time left side of the third dorsal vertebra. Five arteries arise from the arch—viz., two coronaries, for the supply of the muscular tissue of the heart itself; the innominata; and the left carotid and left subclavian arteries. At the commencement of the arch arc three small swellings or pouches, the aortic sinuses, below which are the three semilunar valves or folds of the lining c d membrane, which prevent regurmitation of the blood back 1 .. e into the heart. The thoracic A. extends from the third .
dorsal vertebra to the diaphragm, gradually getting into 61.....--.
the middle line of the spine. The thoracic A. gives off 2 .,, ;.
the bronchial arteries (two or three) to supply the tissue of the lungs; and some small branches (three or four) to -.... • the cesopliagus, and interc-Istal arteries, to supply the walls of the chest (ten on left, and nine on right side). 8'
The abdominal A. passes from the diaphragm to the4 .., - 7.- fourth lumbar vertebra, opposite the lower margin of which it divides into the two common iliac trunks. The abdominal A. gives off the two phrenic arteries to the g" li ::, diaphragm; the cceliae axis, which divides into three large .. Y branches for the stomach, liver, and spleen; the superior mesenteric for the small, and part of the large intestine; : the renals (two); the supra-renals (two), one for each ...
kidney; the spermatic; the inferior mesenteric, for the part of the large intestine not supplied by the superior or . i 0 mesenteric; and four or five. lumbar arteries, which sup- ply the lower part of the abdominal walls (the loins). Where the A. bifurcates, a small artery, the media, or caudal artery, arises, and passes along in the ' , middle line; in fish and in animals with large tails, this branch is a continuation of the A. - ? , 97.
The above is the usual arrangement; but occasionally e • it varies, especially iu the number of arteries springing from the arch. The structure of the A. will be given • under ARTERY; and the comparative anatomy under