3 i8 THE VASCULAR SYSTEM 



Owing to the cardiac condition, the patient is usually in 

 the dorsal decubitus or else in the semi-sitting posture, and 

 the fluid therefore accumulates in the lower limit of the 

 pleural sac posteriorly. 



The Ascending Aorta begins at the aortic orifice of the 

 left ventricle and passes upwards, slightly forwards and to the 

 right. Throughout its course it lies entirely behind the sternum 

 and it approaches most nearly to the anterior surface of the 

 body at its termination, which lies behind the right half of 

 the sternum opposite the second costal cartilage. As the blood 

 enters the ascending aorta from the left ventricle, it impinges 

 on its right wall, and, as a result of this continually recurring 

 pressure, the vessel is rendered oval instead of circular on 

 transverse section. There is thus a normal dilatation, which, 

 under certain circumstances, may become increased so as to 

 constitute a pathological condition. 



Anteriorly ', the ascending aorta is covered by the thin 

 anterior borders of both lungs. When it is the site of aneur- 

 ismal dilatation, it compresses the right lung and projects 

 beyond the right border of the sternum. In this case, visible 

 pulsations may be present in the right second intercostal space 

 and the aortic sounds are heard with maximum intensity in that 

 situation. As it bulges to the right, the ascending aorta may 

 not only compress the right lung but also the superior vena 

 cava, which lies along its right side and on a slightly posterior 

 plane (Fig. 105). 



Close to its origin, the aortic wall presents three small dilata- 

 tions, which are termed the aortic sinuses (of Valsalva). Each 

 sinus is situated opposite a cusp of the aortic valve, and the 

 right and left coronary arteries arise from the anterior and left 

 posterior sinuses, respectively (p. 310). 



The Arch of the Aorta commences at the termination of 

 the ascending aorta and, arching upwards, backwards and to 

 the left, it reaches the left side of the body of the fourth 

 thoracic vertebra, where it becomes continuous with the 

 descending thoracic aorta. The backward inclination of the 



