BRANCHES FROM THE ARCH OF THE AORTA. 195 



the same instant, whereby the blood is thrown into the ventri- 

 cles, and as immediately afterwards the ventricles contract si- 

 multaneously also, whereby the blood is forced into the aorta apd 

 the pulmonary artery, so it is the contraction of ihe ventricles 

 which causes the heart to strike against the parietes of the tho- 

 rax. For, as was first pointed out by Dr. W. Hunter, the blood 

 which is forced through the large arteries, by extending them 

 diminishes their curvature, or brings them more into a straight 

 line, in which effort the heart bounds up from the tendinous cen- 

 tre of the diaphragm. The rilling of the auricles, while this is 

 going on, also assists in protruding the heart forwards. The 

 French anatomists assert, that during the contraction of the ven- 

 tricles, their extremity is elevated or bent upwards on the body 

 of the heart, which will also increase the momentum of the stroke 

 against the thorax. 



CHAPTER IL 



OF THE ARTERIES. 



SECT. I. THE AORTA AND THE BRANCHES FROM ITS CURVATURE. 



THE Aorta is the trunk of the arterial system. Having arisen 

 from the superior posterior end of the left ventricle, its root 

 passes beneath the pulmonary artery, and is entirely concealed 

 in front by it. Keeping to the right, it emerges at the base of 

 the heart, between the right auricle and the trunk of the pulmo- 

 nary artery, being bounded on the right side by the descending 

 cava. Continuing its ascent, it forms a curvature with the con- 

 vexity upwards, and the summit of which rises to within eight 

 or twelve lines of the superior edge of the sternum. This cur- 

 vature is in front of the third and fourth dorsal vertebrae, and its 

 direction is nearly marked out by a line drawn from the anterior 

 extremity of the third right rib, to the posterior end or tubercle 

 of the third one on the left side. Jn this course, therefore, the 



