542 THE ARTERIES. 



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, 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 the aorta (sinuses of 

 Valsalva), opposite to Avhich are attached the three semilunar valves, which serve 

 the purpose of preventing any regurgitation of blood into the cavity of the 

 ventricle. A section of the aorta opposite this part has a somewhat triangular 

 figure, but below the attachment of the valves it is circular. This portion of the 

 aorta is contained in the cavity of the pericardium, and, together with the pul- 

 monary artery, is invested in a tube of serous membrane, continued on to them 

 from the surface of the heart. 



Relations. The ascending aorta is covered at its commencement by the 

 trunk of the pulmonary artery and the right auricular appendix, and, higher up, is 

 separated from the sternum by the pericardium over which lie : the right pleura, 

 and anterior margin of right lung, some loose areolar tissue, and the remains of 

 the thymus gland; behind, it rests upon the right pulmonary artery and left 

 auricle. On the right side it is in relation with the superior vena cava and right 

 auricle ; on the left side, with the main pulmonary artery. 



PLAN OF THE RELATIONS OF THE ASCENDING AORTA. 



In front. 



Pulmonary artery. 

 Right auricular appendix. 

 Pericardium. 



Right side. /ArchofAortaX Leftside. 



Superior cava. ( ^ortioL^ Pulmonary artery. 



Right auricle. j 



Behind. 



Eight pulmonary artery. 

 Left auricle. 

 Pericardium. 



BRANCHES OF THE ASCENDING AORTA. 



The only branches of the ascending aorta are the two coronary arteries. 

 They supply the heart, and are two in number, right and left, arising near the 

 commencement of the aorta immediately above the free margin of the semilunar 

 valves. 



THE CORONARY ARTERIES. 



The Right Coronary Artery, about the size of a crow's quill, arises from the 

 aorta immediately above the free margin of the anterior semilunar valve. It 

 passes forward between the pulmonary artery and the right auricular appendix, 

 then runs obliquely to the righTside in lhe"groove between the right auricle and 

 ventricle, and, curving around the right border of the heart, runs along its posterior 

 surface as far as the posterior interventricular groove, where it divides into two 

 branches, one of which (transverse] continues onward in the groove between the 

 left auricle and ventricle, and anastomoses with the left coronary ; the other (de- 

 scending} descends along the posterior interventricular furrow, supplying branches 

 to both ventricles and to the septum, and anastomosing at the apex of the heart 

 with the descending branches of the left coronary. 



This vessel sends a large branch (marginal) along the thin margin of the right 

 ventricle to the apex, and numerous small branches to the right auricle and ven- 

 tricle, and the commencement of the pulmonary artery (infundibular). 



The Left Coronary, larger than the former, arises immediately above the free 



