634 THE VASCULAR SYSTEMS 



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 for- 

 ward close to the outer side of the innominate vein. In the upper part of the thorax it lies behind 

 the costal cartilages and Internal intercostal muscles, and is crossed by the terminations of the 

 upper six intercostal nerves. Behind it lies upon the pleura, as far as the third costal cartilage; 

 below this level upon the Triangularis sterni muscle. It is accompanied by two venae comites; 

 these unite into a single vein, which passes to the inner side of the artery and ends in the corre- 

 sponding innominate vein. 



Branches. The branches of the internal mammary are: 



Comes nervi phrenici. Anterior intercostal. 



Mediastinal. Perforating. 



Pericardiac. Musculophrenic. 



Sternal. Superior epigastric. 



The comes nervi phrenici (a. pericardiacophrenica) is a long slender branch 

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

 the Diaphragm to which it is distributed; it anastomoses with the other phrenic 

 branches from the internal mammary and with phrenic branches of the abdominal 

 aorta. 



The mediastinal branches (aa. mediastinales anteriores) are small vessels which 

 are distributed to the areolar tissue and lymph nodes in the anterior medias- 

 tinum and to 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 musculophrenic artery. 



The sternal branches (rami sternales) 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 (rami intercostales) 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, the larger, lying near 

 the lower margin of the rib above, and the other, the smaller, 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, reach the Pectoral muscles and the mammary gland. 



The perforating arteries (rami perforantes') correspond to the five or six upper 

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

 the intercostal spaces, and, curving outward, sjipply the Pectoral is 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 musculophrenic artery (a. musculophrenica) is directed obliquely down- 

 ward and outward, behind the cartilages of the false ribs, perforating the Dia- 

 phragm at the eighth or ninth rib, and terminating, considerably reduced in size, 

 opposite the last intercostal space. It gives off anterior intercostal arteries to 

 each of the intercostal spaces across which it passes; these diminish in size as the 

 spaces decrease in length, and are distributed in a manner precisely similar to 

 the anterior intercostals from the internal mammary. The musculophrenic also 



