476 



THE BLOOD-VASCULAR SYSTEM. 



the aorta, and the descending aorta, which last is again divided into thoracic aorta 

 and abdominal aorta, from the position of these parts. 



THE ASCENDING AORTA. 



The ascending aorta is about two inches in length. It commences at the upper 

 part of the left ventricle, on a level with the lower border of the third costal carti- 

 lage, 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 



Right vagus. 

 Recurrent laryngeal. 



Left vagus. 



Left phrenic. 



Thoracic duct. 



Fio. 280. The arch of the aorta and its branches. 



second right costal cartilage, describing a slight curve in its course, and being situ- 

 ated, when distended, about a quarter of an inch behind the posterior surface of the 

 sternum. A little above its commencement it is somewhat enlarged, and presents 

 three small dilatations, called the sinuses of Valsalva, opposite to which are attached 

 the three semilunar valves, which serve the purpose of preventing any regurgitation 

 of blood into the cavity of the ventricle. These valves are placed one in front and 

 two behind. At the union of the ascending with the transverse part of the aorta 

 the calibre of the vessel is increased, owing to a bulging outward of its right wall. 

 This dilatation is termed the great sinus of the aorta. A section of the aorta 



