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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
of the ascending aorta, as far as the under surface of the aortic arch, where it divides, about the level of the fibrocartilage between the fifth and sixth thoracic vertebræ, into right and left branches of nearly equal size.

Relations.—The whole of this vessel is contained within the pericardium. It is enclosed with the ascending aorta in a single tube of the visceral layer of the serous pericardium, which is continued upward upon them from the base of the heart. The fibrous layer of the pericardium is gradually lost upon the external coats of the two branches of the artery. In front, the pulmonary artery is separated from the anterior end of the second left intercostal space by the pleura and left lung, in addition to the pericardium; it rests at first upon the ascending aorta, and higher up lies in front of the left atrium on a plane posterior to the ascending aorta. On either side of its origin is the auricula of the corresponding atrium and a coronary artery, the left coronary artery passing, in the first part of its course, behind the vessel. The superficial part of the cardiac plexus lies above its bifurcation, between it and the arch of the aorta.
  The right branch of the pulmonary artery (ramus dexter a. pulmonalis), longer and larger than the left, runs horizontally to the right, behind the ascending aorta and superior vena cava and in front of the right bronchus, to the root of the right lung, where it divides into two branches. The lower and larger of these goes to the middle and lower lobes of the lung; the upper and smaller is distributed to the upper lobe.
  The left branch of the pulmonary artery (ramus sinister a. pulmonalis), shorter and somewhat smaller than the right, passes horizontally in front of the descending aorta and left bronchus to the root of the left lung, where it divides into two branches, one for each lobe of the lung.
  Above, it is connected to the concavity of the aortic arch by the ligamentum arteriosum, on the left of which is the left recurrent nerve, and on the right the superficial part of the cardiac plexus. Below, it is joined to the upper left pulmonary vein by the ligament of the left vena cava.
  The terminal branches of the pulmonary arteries will be described with the anatomy of the lungs.
 
2. The Aorta
 
  The aorta is the main trunk of a series of vessels which convey the oxygenated blood to the tissues of the body for their nutrition. It commences at the upper part of the left ventricle, where it is about 3 cm. in diameter, and after ascending for a short distance, arches backward and to the left side, over the root of the left lung; it then descends within the thorax on the left side of the vertebral column, passes into the abdominal cavity through the aortic hiatus in the diaphragm, and ends, considerably diminished in size (about 1.75 cm. in diameter), opposite the lower border of the fourth lumbar vertebra, by dividing into the right and left common iliac arteries. Hence it is described in several portions, viz., the ascending aorta, the arch of the aorta, and the descending aorta, which last is again divided into the thoracic and abdominal aortæ.

the Ascending Aorta (Aorta Ascendens) (Fig. 505).—The ascending aorta is about 5 cm. in length. It commences at the upper part of the base of the left ventricle, on a level with the lower border of the third costal cartilage behind the left half of the sternum; it passes obliquely upward, forward, and to the right, in the direction of the heart’s axis, as high as the upper border of the second right costal cartilage, describing a slight curve in its course, and being situated, about 6 cm. behind the posterior surface of the sternum. At its origin it presents, opposite the segments of the aortic valve, three small dilatations called the aortic sinuses. At the union of the ascending aorta with the aortic arch the caliber of the vessel is increased, owing to a bulging of its right wall. This dilatation is termed the bulb of the aorta, and on transverse section presents a somewhat

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