THE INTERNAL MAMMARY ARTERY. 763 



2. The Internal Mammary Artery. — The internal mammary artery (a. mam- 

 maria interna) (Figs. 692, 703) arises from the lower surface of the subclavian, 

 usually a few millimetres lateral to the origin of the vertebral. It is at first directed 

 downward, inward and slightly forward to reach the posterior surface of the first 

 costal cartilage, about half-an-inch lateral to the border of the sternum, and is thence 

 continued vertically downward upon the inner surface of the anterior thoracic wall 

 to the sixth intercost^ space, opposite which it terminates by dividing into the 

 musculo-phrenic and superior epigastric arteries. 



In the upper part of its course the artery rests upon the; dome of the pleura, 

 crosses the posterior surface of the subclavian vein, and is crossed obliquely from 

 above downward and inward by the phrenic nerve. In the thorax it is in con- 

 tact behind with the parietal layer of the pleura as far down as the third costal cart'- 

 lage, and below that with the triangularis sterni muscle. Anteriorly it rests upon 

 the posterior surfaces of the upper five costal cartilages, and, in the intercostal 

 spaces, upon the anterior portions of the internal intercostal muscles. 



Branches. — The internal mammary giv'es off the following branches: (i) \\\e superior 

 phrenic, or comes 7iervi phrenici. (2) the mediastinal branches. (3) the anterior intercostals, 

 (4) the a«/,f/7t»r /^;/c>r()'/'?;/^ branches and the two terminal branches, (5) the nmsculo-phrenic , 

 and (6) the superior epigastric. 



(a) The superior phrenic artery or comes nervi phrenici (a. pericardiacoplirenica) arises from 

 the upper part of the internal mammary, and is a long, slender branch which accompanies the 

 phrenic nerve to the diaphragm, where it anastomoses with the inferior phrenic and musculo- 

 phrenic vessels. In its course it gives off numerous small branches to the pleura and peri- 

 cardium, which anastomose with the mediastinal branches and the bronchial vessels from the 

 thoracic aorta. 



(d) The mediastinal branches (aa. mediastinales anteriores) area number of small vessels 

 which are distributed to the sternum, the remains of the thymus gland, the pericardium, and the 

 adipose tissue of the anterior mediastinum. 



(c) The anterior intercostal arteries (rami intercostales) arise from the internal mammarj' 

 opposite eachjaf the five upper intercostal spaces, and are two in number for each space. They 

 pass outward and slightly downward upon the posterior surface of the intercostal muscles, one 

 along the upper border of each of the intercostal spaces concerned and the other along its lower 

 border, and after having pierced the internal intercostal muscles, they terminate by becoming 

 continuous with the upper and lower divisions respectively of the intercostal branches of the 

 superior intercostal artery and of the three uppermost aortic intercostals. These branches really 

 represent ventral prolongations of the aortic intercostal arteries from which arose the upward 

 and downward branches whose anastomosis resulted in the formation of the internal mammary 

 (compare page 84S). 



{d) The anterior perforating branches (rami perforantes) arise from the internal mam- 

 mary, one opposite each intercostal space that it crosses, and represent the ventral ends of the 

 original aortic intercostal. They pierce the internal intercostal muscles, the anterior intercostal 

 membrane, and the pectoralis major, to supply branches to the sternum and to the integument. 

 The arteries of the third and fourth intercostal spaces are larger than the others and send 

 branches to the mammarv gland. 



(e) The musculo-phrenic artery (a. .musculophrenica) is the lateral terminal branch of the 

 internal mammary. It arises opposite the anterior end of the sixth intercostal space and passes 

 downward and outward along the attachments of the diaphragm to the seventh and eighth costal 

 cartilages, and then, piercing the diaphragm, is continued onward upon the under surface of that 

 muscle to the level of the tenth or eleventh rib. where it terminates by anastomosing with the 

 inferior phrenic arteries and with the ascending branch of the deep circumflex iliac. In addition 

 to branches to the diaphragm, it gives off two anterior intercostal branches opposite each of the 

 intercostal spaces that it crosses as far down as the ninth ; these branches have the same 

 arrangement and significance as the anterior intercostal branches of the internal mammary. 



( /) The superior epigastric artery (a epigastrica superior) is the medial terminal branch 

 of the internal mammary. It continues the course of that artery downward, and passes through 

 the diaphragm in the interval between its costal and sternal origins and enters the sheath of 

 the rectus abdominis. Lower down it passes into the substance of that muscle, where it termi- 

 nates by anastomosing with branches of the deep epigastric artery. 



Anastomoses. — By ineans of its terminal branches the internal mammary 

 makes a double anastomosis in the anterior abdominal walls with branches from the 

 iliac vessels, — namely, with the deep epigastric and deep circumflex iliac branches of 



