326 DISSECTION OF THE THORAX. 



As the right and left branches of the pulmonary artery pass outwards, 

 they cross the air-tubes resulting from the division of the trachea, and 

 inclose with them a lozenge-shaped space which contains some bronchial 

 glands. 



Ductus arteriosus. The ligamentous structure was the continuation in 

 the foetus of the trunk of the pulmonary artery, and was larger than either 

 branch to the lung. At that period the vessel receives the name arterial 

 canal or duct (ductus arteriosus, Botalli), and opens into the aorta rather 

 beyond the origin of the last great vessel of the head and neck from the 

 arch. 



As the lungs do not give passage to the circulating fluid before birth, 

 the impure blood in the pulmonary artery passes through the arterial duct 

 into the aorta below the attachment of the vessels of the head and neck, 

 in order that it may be transmitted to the placenta to be purified. But 

 after birth, when the function of the lungs is established, the current of 

 blood is directed along the branches of the pulmonary artery instead of 

 through the arterial duct ; and this tube becoming gradually smaller, is 

 obliterated before the eighth or tenth day, and forms finally the ligament 

 of the arterial duct. 



The AORTA is the great systemic vessel which conveys the blood from 

 the heart to the different parts of the body. It arches backwards at first 

 to reach the spinal column, and is continued on the spine through the chest 

 and abdomen. In the thorax the vessel is divided into two parts arch 

 of the aorta, and the descending or thoracic aorta. 



Arch of the aorta (fig. 105). The aorta has its origin in the left ventri- 

 cle, and curving backwards over the windpipe and the gullet, forms an 



Fig. 105. 



a. Aortic arch. 



b. Innominate artery. 



c. Left common carotid. 



d. Lett subclavian. 



e. Ligamentum ductus arteriosi. 

 /. Vena cava superior. 



ff. Left innominate vein. 

 h. Right innominate vein. 



f. Left upper intercostal vein, 

 fc. Vena azygos major. 



I. Left subclavian vein. 

 n. Thoracic duct. 

 o. Coronary artery. 



P 



ARCH OF THE AORTA AXD ITS GREAT VESSELS. 



arch which ceases on the left side of the spinal column, at the lower bor- 

 der of the fifth dorsal vertebra. The arch has its convexity upwards and 

 to the right, and its concavity to the root of the left lung ; and from it the 

 large vessels for the supply of the upper part of the body take their origin. 

 For the purpose of reducing to order the numerous connections of this 



