478 THE BLOOD-VASCULAR SYSTEM. 



verse branch of the right coronary ; the other (descending] passes along the anterior 

 interventricular groove to the apex of the heart, where it anastomoses with the de- 

 scending branches of the right coronary. The left coronary supplies the left 

 auricle and its appendix, gives branches to both ventricles, and numerous small 

 branches to the pulmonary artery and commencement of the aorta. 1 



Peculiarities. These vessels occasionally arise by a common trunk, or their number may 

 be increased to three, the additional branch being of small size. More rarely there are two 

 additional branches. 



THE ARCH OF THE AORTA. 



The arch, or transverse aorta, commences at the upper border of the second 

 chondro-sternal articulation of the right side, and passes at first upward and back- 

 ward and from right to left, and then from before backward, to the left side of the 

 lower border of -the fourth dorsal vertebra behind. Its upper border is usually 

 about an inch below the upper margin of the sternum. 



Between the origin of the left subclavian artery and the attachment of the 

 ductus arteriosus the lumen of the foetal aorta is considerably narrowed, forming 

 what is termed the aortic isthmus, while immediately beyond the ductus arte- 

 riosus the vessel presents a fusiform dilatation which His has named the aortic 

 spindle the point of junction of the two parts being marked in the concavity 

 of the arch by an indentation or angle. These conditions persist, to some extent, 

 in the adult, where His found that the average diameter of the spindle exceeded 

 that of the isthmus by 3 mm. (about one-eighth of an inch). 



Relations. Its .anterior surface is covered by the pleurae and lungs and the 

 remains of the thymus gland, and crossed toward the left side by the left pneumo- 

 gastric and phrenic nerves and superficial cardiac branches of the left sympathetic 

 and vagus, and by the left superior intercostal vein. Its posterior surface lies on 

 the trachea, just above its bifurcation, on the great, or deep, cardiac plexus, the 

 oesophagus, thoracic duct, and left recurrent laryngeal nerve. Its upper border is 

 in relation with the left innominate vein, and from its upper part are given off the 

 innominate, left common carotid and left subclavian arteries. Its Lower border is 

 in relation with the bifurcation of the pulmonary artery, the remains of the ductus 

 arteriosus, which is connected with the left division of that vessel, and the super- 

 ficial cardiac plexus ; the left recurrent laryngeal nerve winds round it from 

 before backward, whilst the left bronchus passes below it. 



PLAN OF THE RELATIONS OF THE ARCH OF THE AORTA. 



Above. 



Left innominate vein. 

 Innominate artery. 

 Left carotid. 

 Left subclavian. 

 In From. Behind. 



Pleurae and lungs. f \^ Trachea. 



Remains of thymus gland. / Arch \ Deep cardiac plexus. 



Left pneumogastric nerve. Ol (Esophagus. 



Left phrenic nerve. 1 Thoracic duct. 



Superficial cardiac nerves. / Left recurrent nerve. 

 Left superior intercostal vein. 



Below. 



Bifurcation of pulmonary artery. 

 Remains of ductus arteriosus. 

 Superficial cardiac plexus. 

 Left recurrent nerve. 

 Left bronchus. 



1 According to Dr. Samuel West, there is a very free and complete anastomosis between the two 

 coronary arteries (Lancet, June 2. 1883, p. 945). This, however, is not the view generally held by 

 anatomists, for, with the exception of the anastomosis mentioned above in the auriculo-ventricular and 

 interventricular grooves, it is believed that the two arteries only communicate by very small vessels 

 ;n the substance of the heart. 



