526 THE BLOOD-VASCULAR SYSTEM. 



The posterior scapular passes beneath the Levator anguli scapulae to the supe- 

 rior angle of the scapula, and then descends along the posterior border of that bone 

 as far as the inferior angle. In its course it is covered by the Rhomboid muscles, 

 supplying them and the Latissimus dorsi and Trapezius, and anastomosing with the 

 suprascapular and subscapular arteries, and with the posterior branches of some of 

 the intercostal arteries. 



Peculiarities. The superficial cervical frequently arises as a separate branch from the 

 thyroid axis ; and the posterior scapular, from the third, more rarely from the second, part of 

 the subclavian. 



The Internal mammary (Fig. 301) arises from the under surface of the first 

 portion of the subclavian artery, opposite the thyroid axis. It passes downward 

 and inward behind the costal cartilage of the first rib to the inner surface of the 

 anterior wall of the chest, resting against the costal cartilages about half an inch 

 from the margin of the sternum ; and, at the interval between the sixth and seventh 

 cartilages, divides into two branches, the musculo-phrenic and superior epigastric. 



Relations. At its origin it is covered by the internal jugular and subclavian 

 veins, and as it enters the thorax is crossed from without inward by the phrenic 

 nerve, and then passes forward close to the outer side of the innominate vein. In 

 the upper part of the thorax it lies behind the costal cartilages and Internal inter- 

 costal muscles, and is crossed by the terminations of the upper six intercostal nerves. 

 At first it lies upon the pleura, but at the lower part of the thorax the Triangularis 

 sterni separates the artery from this membrane. It has two venae comites ; these 

 unite into a single vein, which joins the innominate vein of its own side. 



The branches of the internal mammary are 



Comes Nervi Phrenici (Superior Phrenic). Anterior Intercostal. 



Mediastinal. Perforating. 



Pericardiac. Musculo-phrenic. 



Sternal. Superior Epigastric. 



The comes nervi phrenici (superior phrenic), is a long slender branch which 

 accompanies the phrenic nerve, between the pleura and pericardium, to the 

 Diaphragm, to which it is distributed, anastomosing with the other phrenic 

 arteries from the internal mammary and abdominal aorta. 



The mediastinal branches are small vessels which are distributed to the areolar 

 tissue and lymphatic glands in the anterior mediastinum and the remains of the 

 thymus gland. 



The pericardiac branches supply the upper part of the anterior surface of the 

 pericardium, the lower part receiving branches from the musculo-phrenic artery. 



The sternal branches are distributed to the Triangularis sterni and to the 

 posterior surface of the sternum. 



The mediastinal, pericardiac, and sternal branches, together with some twigs 

 from the comes nervi phrenici, anastomose with branches from the intercostal and 

 bronchial arteries, and form a minute plexus beneath the pleura, which has been 

 named by Turner the subpleural mediastinal plexus. 



The anterior intercostal arteries supply the five or six upper intercostal spaces. 

 The branch corresponding to each space soon divides into two, or the two 

 branches may come off separately from the parent trunk. The small vessels pass 

 outward in the intercostal spaces, one lying near the lower margin of the rib 

 above, and the other near the upper margin of the rib below, and anastomose 

 with the intercostal arteries from the aorta. They are at first situated between 

 the pleura and the Internal intercostal muscles, and then between the Internal 

 and External intercostal muscles. They supply the Intercostal muscles, and, by 

 branches which perforate the External intercostal muscle, the Pectoral muscles 

 and the mammary gland. 



The perforating arteries correspond to the five or six upper intercostal spaces. 

 They arise from the internal mammary, pass forward through the intercostal 

 spaces, and, curving outward, supply the Pectoralis major 'and the integument. 



