884 THE VASCULAR SYSTEM. 



the aortic arch, to which it is connected by the ligamentum arteriosum, and the left re- 

 current nerve ; below, it is in relation with the lower left pulmonary vein. After entering 

 the lung it descends, like the right pulmonary artery, posterior and lateral to the stem 

 bronchus, and between its ventral and dorsal branches. 



Branches. Just before it passes through the hilum it gives off a branch to the 

 upper lobe of the left lung, and in the substance of the lung its branches correspond 

 with the ventral, dorsal, and accessory branches of the bronchial tube. 



THE SYSTEMIC ARTERIES. 



AORTA. 



The aorta is the main trunk of the general arterial system. It commences at the 

 base of the left ventricle and ascends, with an inclination to the right, to the level 

 of the second right costal cartilage ; then it curves backwards and to the left, until 

 it reaches the left side of the lower border of the fourth thoracic vertebra ; there 

 it turns downwards and descends, through the thorax into the abdomen, where it 

 terminates, on the left of the median plane, at the level of the fourth lumbar 

 vertebra, by bifurcating into the two common iliac arteries. The portion of the 

 aorta which is situated in the thorax is, for convenience, termed the thoracic aorta, 

 and the rest of the vessel is known as the abdominal aorta. 



AORTA THORACALIS. 



The thoracic aorta is subdivided into aorta ascendens, arcus aortse, and aorta 

 descendens. 



Aorta Ascendens. The ascending aorta lies in the middle mediastinum. It 

 springs from the base of the left ventricle, posterior to the left margin of the sternum 

 opposite the lower border of the third left costal cartilage and at the level of thf 

 body of the sixth thoracic vertebra. From its origin it passes upwards, anteriorly 

 and to the right, and it terminates in the arch of the aorta, posterior to the righi 

 margin of the sternum, at the level of the second costal cartilage. Its lengtl 

 is from 50 to 56 mm. (2 to 2J inches), and its diameter is 28 mm. (1^ inches) 

 In the adult it is a little narrower at its commencement than the pulmonary 

 artery is, but in old age it enlarges and exceeds the latter vessel in size. Th< 

 diameter, however, is not uniform throughout the whole length of the ascending 

 aorta. Its dilated commencement, the bulbus aortse, has three secondary dilatations 

 the sinus aortse (Valsalva) in its wall, immediately above the semilunar cusps o 

 the aortic valve ; one is anterior in position, and two are situated posteriorly. A 

 a higher level there is a diffuse bulging of the right wall, which is known as th 

 great sinus of the aorta. 



Relations. The ascending aorta is completely enclosed within the fibrous per. 

 cardium which blends above with the sheath of the vessel, and it is enveloped, togethe 

 with the stem of the pulmonary artery, in a tubular prolongation of the serous pericai 

 dium. At its origin it has the pulmonary artery in front, the transverse sinus of th 

 pericardium and the anterior wall of the left atrium behind, and the right atrium on it 

 right side. In the upper part of its course the ascending aorta is overlapped by th , 

 anterior margins of the right lung and right pleural sac, whilst posterior to it are th 

 right atrium, the right branch of the pulmonary artery, the right bronchus, and the lei 

 margin of the superior vena cava. The superior vena cava lies on the right side, an 

 partly posterior to the upper part of the ascending aorta, whilst the pulmonary artery 

 at first anterior to it and then, at a higher level, on its left side. 



Branches. Two branches arise from the ascending aorta, viz., the right and the le 

 coronary arteries. The right coronary artery springs from the anterior, and the le 

 from the left posterior sinus of the aorta (Valsalva) (Fig. 751). 



Arcus Aortse. The arch of the aorta lies in the superior mediastinum, posteric 

 to the lower part of the manubrium sterni, and connects the ascending with tl 

 descending aorta. It commences posterior to the right margin of the sternum, c 

 a level with the second costal cartilage, and extends to the left side of the low< 



