The aorta, after having passed the diaphragm is thus named. It rests on the bodies of the vertebras, extending from the front of the last dorsal to the fourth lumbar vertebra, a little to the left of the median line, where it usually divides. The anterior surface of the great artery is successively in apposition with the liver, the splenic vein, the pancreas, the third portion of the duodenum, the left renal vein, and the peritoneum. The vena cava lies at its right side, the right crus of the diaphragm being interposed at the upper part of the abdomen ; close to the same side, are the thoracic duct and the azygos vein, which are placed between the aorta and the right crus of the diaphragm. The aorta is surrounded by a mesh of nerves derived from the sympathetic.
The abdominal aorta, [aorta abdominalis, French : aorte abdominale], gives numerous branches, which may be divided into two sets, viz., those which supply the viscera and and those which are distributed to the walls of the abdomen. The former consist of the coeliac artery, the superior mesenteric , the inferieur mesenteric, the capsular, the renal, and the spermatic arteries ; which among the latter are included the phrenic, the lumbar, and the middle sacral arteries. The first three of the visceral branches are single arteries.
The internal iliac artery [a. iliaca interna; French: Artère iliaque interne ; Syn: hypogastrica, pelvica], short and thick trunk, separates from the external iliac immediately after its origin, and dips into the pelvis to supply the walls and the viscera of that cavity.
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The suprarenal or capsular arteries [a. suprarenales], are two very small vessels which arise from the aorta on a level with the superior mesenteric artery, and incline obliquely outwards upon the crura of the diaphragm to reach the under surface of the suprarenal capsules, to which bodies they are distributed, anastomosing at the same time with the other capsular branches derived from the phrenic and the renal arteries. In the foetus these arteries are of large size.
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The common iliac artery, extending in a line from the left side of the umbilicus towards the middle of Poupart's ligament, and being placed at its commencement on a level with the highest part of the crest of the ilium, may be approached in an operation, by dividing the abdominal muscles to a sufficient extent, in the iliac region and a little above this part of the abdomen.
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The abdominal aorta ends by dividing into two trunks, named the common iliac arteries. The bifurcation usually takes place on the body of the fourth lumbar vertebra, a little to the left of the middle line. The point here indicated will be found nearly on a level with a line drawn from one crista of the ilium to the other, and is opposite to the left side of the umbilicus. It should, however, be observed, that the place of division is very inconstant in its position, as will be seen from the following statement.
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Besides slight differences between the arteries of the two sides, in length and direction, by no means of constant occurrence, the common iliac arteries vary in their place of origin, and in the point at which they divide.
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A very remarkable case is recorded of the existence of a large pulmonary branch which arose from the abdominal aorta, close to the coeliac artery, and after passing upwards through the oesophageal opening in the diaphragm, divided into two branches, which were distributed to the lungs, near their bases.
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The common iliac arteries, [a. iliacae communes], commencing at the bifurcation of the aorta, pass downwards and outwards, diverging from each other, and divide opposite the intervertebral substance between the last lumbar vertebra and the sacrum, into two branches, named the internal and external iliac arteries — the former being distributed to the walls and viscera of the pelvis, whilst the latter is prolonged into the lower limb, after having sent two important branches to the walls of the abdomen.
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The middle sacral artery, [a. sacralis media], the last of the branches of the abdominal aorta, is a small vessel, about, the size of a crowquill, which arises from the extremity of the aorta, just at its bifurcation. From this point the artery proceeds downwards upon the last lumbar vertebra and over the middle of the sacrum, as far as the coccyx, where it forms small arches of anastomosis with the lateral sacral arteries. From its anterior surface some small branches come forwards within the fold of the mesorectum, and ramify upon the posterior surface of the intestine, and on each side others spread out upon the sacrum, and anastomose with the lateral sacral arteries, occasionally sending small offsets into the anterior sacral foramina. The middle sacral artery sometimes deviates a little to the side, and proceeds, not from the bifurcation of the aorta, but from one of the common iliac arteries, usually from that of the left side. This artery represents the caudal prolongation of the aorta of animals having a tail.
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