After having crossed the diaphragmatic channel, the aorta belongs to the abdominal area. Applied on the vertebral level, it is located behind the intestinal mass. The abdominal aorta moves vertically in bottom however, the diaphragmatic opening being a little on the left of the line of centers, one can say that the abdominal aorta direction continues the thoracic aorta , and only on the level of the fourth lumbar vertebra it becomes exactly median thus moves slightly on the right.
The internal mammary artery, of a lower diameter than the one of the vertebral artery, is remarkable by the extent of its course and by the multiplicity of its branches; it arises from the former face of the subclavian artery, at 3 or 4 mm apart from the vertebral artery. From its origin, it goes bottom, ahead, and a little medialward and reaches the posterior face of the first costal cartilage; there, it becomes vertical, perpendicularly crosses the posterior face of the first six costal cartilages, and, at the level of the sternal end of the sixth intercostal space, is divided into two terminal sections, one intern and the other extern.
The renal arteries, two, arise on the side faces from the abdominal aorta, a little below the anterior mesenteric artery, on the level of the second lumbar vertebra. Their volume is important their diameter can reach 8 millimeters. Generally the renal arteries are detached at the same level, sometimes the left renal artery arises on a higher level. It is traditional to say that the arteries move horizontally outwards but the assertion is not absolutely exact: indeed, these arteries move obliquely bottomward, forming with the aorta an acute angle which can go down up to 45 degrees. In addition to this obliqueness in the frontal plan, the renal artery follows a curve of a posterior concavity which adapts to the convexity of the vertebral body this curve is more marked on the right than on the left.
The arch of the aorta (arcus aortas), commences at the upper part or base of the left ventricle of the heart, in front of the left auriculo-ventricular orifice ; it then passes upwards and to the right side, somewhat in the direction of the heart itself, crossing obliquely behind the sternum, and at the same time approaching more nearly to that bone; having gained the level of the upper border of the second costal cartilage of the right side, the vessel alters its course, and arches over from right to left, at the same time inclining backwards to reach the left side of the second dorsal vertebra ; here it makes another turn, and changes its direction so as to incline downwards upon the left side of the third dorsal vertebra, at the lower border of which the arch of the aorta becomes continuous (without any mark of separation) with the straight descending portion of the vessel known as the thoracic aorta.
The pulmonary artery [a. pulmonalis] passes from the right ventricle to the lungs. It differs from all other arteries in the body in that it contains venous blood. It arises as a short, thick trunk from the conus arteriosus of the right ventricle, and, after a course of about 5 cm. (2 in.) within the pericardium, divides into a right and a left branch. These branches pass to the, right and the left lung respectively.
The external carotid artery gives origin to eight branches, including the two into which it finally divides. For the purposes of description these may be arranged into three sets. 1. Those which are directed forwards, viz., the superior thyroid, the lingual, and the facial. 2. Those which run backwards, — the occipital and posterior auricular; and 3, those which ascend, viz., the ascending pharyngeal branch with the temporal and internal maxillary, — the two terminal branches.
In most parts of the body the description of the artery of one side serves for that of the other likewise ; but this is not the case as regards the subclavian arteries (French : artère sous-clavière), for, as the right subclavian artery commences at the division of the innominate artery, whilst the left subclavian arises at once from the arch of the aorta, it follows that the two vessels must, in the first part of their course, differ materially in their length, direction, and connexions with contiguous parts.
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