THE AORTA. 437 



lating 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 per- 

 fectly 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 ex- 

 tremely tortuous in their course, to accommodate themselves to the movements 

 of the parts. The uterine arteries are also tortuous, to accommodate them- 

 selves 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 sur- 

 face 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 is described in the Introduction. 



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

 the systemic circulation, the aorta, and its branches; and then the efferent 

 trunk of the pulmonic circulation, the pulmonary artery. 



THE AORTA. 



The Aorta (aoptjj; arteria magnet) is the main trunk of a series of vessels, 

 which, arising from the heart, convey the red oxygenated blood to every part 

 of the body for its nutrition. This vessel commences at the upper part of the 

 left ventricle, and, after ascending for a short distance, arches backwards to 

 the left side, over the root of the left lung, descends within the thorax on the 

 left side of the vertebral column, passes through the aortic opening in the 

 Diaphragm, and entering the abdominal cavity, terminates, considerably dimin- 

 ished in size, opposite the fourth lumbar vertebra, where it divides into the 

 right and left common iliac arteries. Hence its subdivision into the arch of the 

 aorta, the thoracic aorta, and the abdominal aorta, from the direction or position 

 of its parts. 



ARCH OP THE AORTA. 



Dissection. In order to examine the arch of the aorta, open the thorax, by dividing the 

 cartilages of the ribs on each side of the sternum, raising this bone from below upwards, and 

 then sawing through the sternum on a level with its articulation with the clavicle. By this 

 means, the relations of the large vessels to the upper border of the sternum and root of the neck 

 are kept in view. 



The Arch of the Aorta extends from the origin of the vessel at the upper 

 part of the left ventricle to the lower border of the body of the fourth dorsal 

 vertebra. At its commencement, it ascends behind the sternum, obliquely 

 upwards and forwards towards the right side, and opposite the upper border of 

 the second costal cartilage of the right side, passes transversely from right to 

 left, and from before backwards, to the left side of the third dorsal vertebra ; it 

 then descends upon the left side of the body of the fourth dorsal vertebra, at 

 the lower border of which it takes the name of thoracic aorta. The arch of 

 the aorta describes a curve, the convexity of which is directed upwards and to 

 the right side; and it is subdivided, at the points where it changes its direction, 

 so as to be described in three portions, the ascending, transverse, and descending 

 portions of the arch of the aorta. 



