540 THE ARTERIES. 



branches. In the limbs the anastomoses are most frequent and of largest size 

 around the joints, the branches of an artery above freely inosculating with 

 branches from the vessels below ; these anastomoses are of considerable interest to 

 the surgeon, as it is by their enlargement that a collateral circulation is established 

 after the application of a ligature to an artery for the cure of aneurism. The 

 smaller branches of arteries anastomose more frequently than the larger, and 

 between the smallest twigs these inosculations become so numerous as to constitute 

 a close network that pervades nearly every tissue of the body. 



Throughout the body generally the larger arterial branches pursue a perfectly 

 straight course, but in certain situations they are tortuous ; thus, the facial artery 

 in its course over the face, and the arteries of the lips, are extremely tortuous in 

 their course, to accommodate themselves to the movements of the parts. The 

 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. 



THE PULMONARY ARTERY (Fig. 344). 



The pulmonary artery conveys the venous blood from the right side of the heart 

 to the lungs. It is a short, wide vessel, about two inches in length, arising from 

 the left side of the base of the right ventricle, in front of the aorta. It passes 

 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 transverse aorta, where 

 it divides 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, being enclosed with it in a tube of serous membrane, 

 continued upward from the base of the heart, and has attached to it, above, the 

 fibrous layer of the membrane. Behind, it rests at first upon the ascending 

 aorta, and higher up lies in front of the left auricle. On each side of its origin 

 is the appendix of the corresponding auricle and a coronary artery ; and higher 

 up it passes to the left side of the ascending aorta. 



The right pulmonary artery, longer and larger than the left, pierces the peri- 

 cardium and 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, which is the smaller, supplies the lower lobe ; the upper supplies 

 the upper lobe, giving a branch to the middle lobe. 



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

 pierces the pericardium and 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 (transverse aorta) by a short fibrous cord, 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. 



