INTERNAL MAMMARY VESSELS. 



239 



Fig. 72. 



Action. The muscle assists in depressing the anterior ends of the ribs ; 

 and by diminishing the size of the thorax, it becomes an expiratory 

 muscle. 



The internal mammary artery is a branch of the subclavian (p. 77), 

 and enters the thorax beneath the cartilage of the first rib. It is con- 

 tinued through the thorax, lying beneath the costal cartilages and 

 about half an inch from the sternum, as far as the interval between the 

 sixth and seventh ribs; there it gives externally a large muscular branch 

 (musculo-phrenic), and passing beneath the seventh rib, enters the sheath 

 of the rectus muscle in the wall 

 of the abdomen. In the chest 

 the artery lies on the pleura 

 and the triangularis sterni, and 

 is crossed by the intercostal 

 nerves. It is accompanied by 

 two veins, and by a chain of 

 lymphatic glands. The follow- 

 ing branches take origin in the 

 thorax : 



a. A small branch (comes 

 nervi phrenici) arises as soon 

 as the artery enters the chest, 

 and descends to the diaphragm 

 along the phrenic nerve. 



b. A few small mediastinal 

 branches are disti'ibuted to the 

 remains of the thymus gland, 

 the pericardium, and the tri- 

 angularis sterni muscle. 



c. Two anterior intercostal 



branches turn outwards in each space, one being placed on the border of 

 each costal cartilage, and terminate by anastomosing with the aortic inter- 

 costal arteries. 



d. Perforating branches, one or two opposite each space, pierce the in- 

 ternal intercostal and pectoral muscles, and are distributed on the surface 

 of the thorax with the anterior cutaneous nerves : the lower branches sup- 

 ply the mamma in the female. 



e. The musculo-phrenic branch courses outwards beneath the cartilages 

 of the seventh and eighth ribs, and enters the wall of the abdomen by 

 perforating the diaphragm : it supplies anterior branches to the lower 

 intercostal spaces. Its termination will appear in the dissection of the 

 abdomen. 



Two veins accompany the artery; these join into one trunk, which 

 opens into the innominate vein. 



The intercostal nerves, seen now in the anterior part of their extent, 

 are the anterior primary branches of the dorsal nerves, and supply the 

 wall of the thorax. Placed at first between the layers of the intercostal 

 muscles, each gives off the lateral cutaneous nerve of the thorax, about 

 midway between the spine and the sternum. Diminished in size by the. 

 emission of that offset, the trunk is continued onwards, at first in, and 

 afterwards beneath the internal intercostal muscle as far as the side of the 

 sternum, where it ends as the anterior cutaneous nerve of the thorax. 

 Branches supply the intercostal muscles, and the triangularis sterni. 



VIEW FROM BEHIND OF THE ATTACHMENTS OF THE 

 TRIANGULARIS STERNI MUSCLE, A. 



