THE ARTERIES. 475 



uterine arteries are also tortuous, to accommodate themselves to the increase of 

 size which the organ undergoes during pregnancy. Again, the internal carotid 

 and vertebral arteries, previous to their entering the cavity of the skull, describe 

 a series of curves, which are evidently intended to diminish the velocity of the 

 current of blood by increasing the extent of surface over which it moves and 

 adding to the amount of impediment which is produced by friction. 



The arteries are dense in structure, of considerable strength, highly elastic, 

 and, when divided, they preserve, although empty, their cylindrical form. 



The minute structure of these vessels has been described in the chapter on 

 General Anatomy. 



In the description of the arteries we shall first consider the efferent trunk of 

 the pulmonic circulation, the pulmonary artery, and then the efferent trunk of the 

 systemic circulation, the aorta and its branches. 



PULMONARY ARTERY (Fig. 280). 



The Pulmonary Artery conveys the venous blood from the right side of the 

 heart to the lungs. It is a short, wide vessel, about 2 inches in length and 1 

 inches (30 mrn.) in diameter, arising from the left side of the base (conus arterio- 

 sus) of the right ventricle, in front of the aorta. It extends obliquely upward 

 and backward, passing at first in front of and then to the left of the ascending 

 aorta, as far as the under surface of the arch, where it divides, about on a level 

 with the intervertebral substance between the fifth and sixth dorsal vertebrae, 

 into two branches of nearly equal size, the right and left pulmonary arteries. 



Relations. The whole of this vessel is contained, together with the ascending 

 aorta, in the pericardium. It is enclosed with the aorta in a single tube of the 

 serous pericardium, which is continued upward upon them from the base of the 

 heart and connects them together. The fibrous layer of the pericardium becomes 

 gradually lost upon the external coat of its two branches. In front, the pulmonary 

 artery is separated from the anterior extremity of the second left intercostal space 

 by the pleura and left lung, in addition to the pericardium ; it rests at first upon 

 the ascending aorta, and higher up lies in front of the left auricle on a plane 

 posterior to the ascending aorta. On each side of its origin is the appendix of 

 the corresponding auricle and a coronary artery, the left coronary artery passing, 

 in the first part of its course, behind the vessel. 



The right pulmonary artery, longer and larger than the left, runs horizontally 

 outward, behind the ascending aorta and superior vena cava, to the root of the right 

 lung, where it divides into two branches, of which the lower and larger supplies 

 the middle and lower lobes ; the upper and smaller is distributed to the upper lobe. 



The left pulmonary artery, shorter and somewhat smaller than the right, 

 passes horizontally in front of the descending aorta and left bronchus to the root 

 of the left lung, where it divides into two branches for the two lobes. 



The root of the left pulmonary artery is connected to the under surface of the 

 arch of the aorta by a short fibrous cord, the ligamentum arteriosum; this is the 

 remains of a vessel peculiar to foetal life, the ductus arteriosus. 



The terminal branches of the pulmonary artery will be described with the 

 anatomy of the lung. 



THE AORTA. 



The aorta (do/>rjy, arteria magna) is the main trunk of a series of vessels which 

 convey the oxygenated blood to the tissues of the body for their nutrition. This 

 vessel commences at the upper part of the left ventricle, where it is about one and 

 one-eighth inches in diameter, and, after ascending for a short distance, arches 

 backward and to the left side, over the root of the left lung, then descends within 

 the thorax on the left side of the vertebral column, passes through the aortic open- 

 ing in the Diaphragm, and, entering the abdominal cavity, terminates, considerably 

 diminished in size (about seven-tenths of an inch in diameter), opposite the lower 

 border of the fourth lumbar vertebra, where it divides into the right and left 

 common iliac arteries. Hence it is divided into the ascending aorta, the arch of 



