196 CIRCULATORY SYSTEM. 



aorta passes over the right pulmonary artery, across the left 

 bronchus, and applies itself to the left side of the spine, about 

 the third or fourth dorsal vertebra. It is this curvature which 

 obtains the name of the Arch of the Aorta, (Arcus Jlortoz.) 



Near its origin, where the aorta is still within the pericardium, 

 it has very commonly, especially in persons advanced in age, a 

 dilatation, which is calledthe great sinus to distinguish it from 

 the lesser sinuses, or those of Valsalva. This dilatation is use- 

 ful in diminishing the resistance arising from the curvature of 

 the aorta, to the current of blood, or rather it is a provision 

 for doing away with the effects of this friction, as by it a larger 

 current of blood becomes a compensation for diminished velo- 

 city. The ascending portion of the arch is to the right of the 

 vertebral column, the descending portion to the left, and the 

 middle or horizontal part goes in front of the trachea. 



The aorta, in its descent down the thorax, is placed in the 

 posterior mediastinum, and is covered on one side by the left 

 pleura, while the other side is in contact with the left surface of 

 the bodies of the dorsal vertebrse. At the lower part of the 

 thorax it inclines towards the middle line of the vertebrae, in 

 order to reach the hiatus aorticus of the diaphragm, through 

 which it penetrates to the abdomen. In the abdomen it de- 

 scends in front of the lumbar vertebras, somewhat on their left 

 side; and at the intervertebral space between the fourth and 

 fifth vertebra) of the loins, or somewhat above it, it ceases, by 

 being divided into two large trunks, the Primitive lliacs ; one 

 for each lower extremity, and the corresponding side of the 

 pelvis. 



In this course of the aorta, from the heart to the loins, it first 

 gives off the branches which supply the head and the superior 

 extremities : then, those which supply the sides of the thorax ; af- 

 terwards in the abdomen, it detaches the trunks which supply 

 the viscera and the sides of the latter cavity. 



The Coronary Arteries are, strictly speaking, the first branches 

 of the aorta, but as they belong especially to the heart, their de- 

 scription is associated with it. In all the space between them 

 and the superior convexity of the aortic arch no branches are 

 given off; but as the aorta is crossing the trachea three conside- 

 rable trunks arise from it, which are distributed upon the head 

 and the upper extremities principally. They are, the Arteria 



