94 ANATOMY FOK NUKSES. [CHAP. IX. 



its speed in the larger and less numerous trunks, just as a river 

 flows more rapidly through its narrow and deep channels, and 

 lingers in those that are broad and shallow. 



The arteries unite at frequent intervals when they are said to 

 anastomose or inosculate. Such inosculations admit of free 

 communication between the currents of the blood, tend to pro- 

 mote equality of distribution and of pressure, and to obviate 

 the effects of local interruption. 



Arteries commonly pursue a tolerably straight course, but in 

 some parts of the body they are tortuous. They often become 

 markedly tortuous in old persons as the walls of the arteries 

 increase in length and size. 



In describing the distribution of the arteries we shall first 

 consider the artery arising from the left ventricle of the heart, 

 the aorta, and its branches. 



The aorta. The aorta is the main trunk of the arterial sys- 

 tem. Springing from the left ventricle of the heart, it arches 

 over the root of the left lung, descends along the vertebral col- 

 umn, and after passing through the diaphragm into the abdomi- 

 nal cavity, ends opposite the fourth lumbar vertebra by dividing 

 into the right and left common iliac arteries. In this course 

 the aorta forms a continuous single trunk, which gradually 

 diminishes in size from its commencement to its termination 

 (from 28 to 17 mm.), and gives off larger or smaller branches 

 at various points. It may be divided into the ascending aorta, 

 the short part which is contained in the pericardium ; the arch, 

 the part extending from the ascending aorta, and forming a 

 well-marked curve in front of the trachea, and around the root 

 of the left lung to the border of the fourth dorsal vertebra ; 

 the descending thoracic aorta, the comparatively straight part 

 extending to the diaphragm; the abdominal aorta, below the 

 diaphragm. The ascending aorta gives off two small branches, 

 the right and left coronary arteries, which supply the substance 

 of the heart with blood. The arch gives off three large trunks, 

 the innominate, the left common carotid, and the left subclavian 

 artery. 



The innominate artery arises from the right upper surface of 

 the arch, ascends obliquely towards the right, until, arriving on 

 a level with the upper margin of the clavicle, it divides into the 

 right common carotid and right subclavian arteries. Its usual 

 length is from one to two inches. 



