96 THE ARTERIAL TREE. 



The aorta, the great trunk of this tree, has its origin in 

 the superior part of the left ventricle of the heart ; ascends 

 behind the pulmonary artery to the right side, on a level 

 with the articulation of the second rib with its cartilage, 

 then crosses transversely about an inch below the upper 

 edge of the sternum to the left side, where it makes a second 

 turn downwards and inwards to the bodies of the third or 

 fourth dorsal vertebra). This course of the aorta describes 

 a curve called its arch, consisting of an ascending, trans- 

 verse, and descending portions. From the arch we follow 

 the aorta descending through the thorax upon the left side 

 of the vertebral column to the diaphragm, beneath the 

 crura of which muscle it passes, and thence continues to 

 descend on the median line to the space between the fourth 

 and fifth lumbar vertebra, where it terminates, dividing 

 into the common iliacs. 



The portion above the diaphragm, beginning with the 

 arch, is called the thoracic aorta the portion below is the 

 abdominal aorta. 



The first branches given off by the aorta after leaving the 

 heart, are the coronary arteries two in number the right 

 and left, which supply the right and left portions of the 

 heart. The next in order are those coming off from the arch, 

 which are the arteria innominata, the left carotid, and the 

 left subclavian. These are large and important branches, 

 supplying, with arterial blood, the head, neck, and superior 

 extremities. The arteria innominata, after proceeding 

 about an inch and a quarter, divides into two branches, the 

 right carotid and the right subclavian. The former passes 

 up the right side of the neck to opposite the corner of the 

 os-hyoides, where it again divides into the external and 

 internal carotid arteries, the former supplying the right 

 side of the face and the right and posterior parts of the 

 head, together with the right upper neck, while the latter 

 enters within the cranium and supplies the right half of 

 the brain. 



The left carotid, arising from the arch of the aorta, 

 pursues a similar course upwards upon the left side of the 



