586 THE ARTERIES. 



beneath the Levator anguli scapulae to the superior angles of the scapula. It now 

 descends along the posterior border of that bone as far as the inferior angle, where 

 it anastomoses with the subscapular branch of the axillary. 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. This arrangement is almost as common as the one already given. 



The Internal Mammary arises from the under surface of the first portion of 

 the subclavian artery, opposite the thyroid axis. It descends 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 crossed by the phrenic nerve. In the upper part of the thorax it lies 

 against the costal cartilages and Internal intercostal muscles in front, and is 

 covered by the pleura behind. At the lower part of the thorax the Triangularis 

 sterni separates the artery from the pleura. 



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 Avhich 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. 

 Those which correspond to the second, third, and fourth spaces are distributed to 

 the mammary gland. In females, during lactation, these branches are of large size. 



The musculo-phrenic artery is directed obliquely downward and outward, 



