THE INTERNAL MAMMARY ARTERY. 391 



branches into which the artery divides, musculo-phrenic, inclines out- 

 wards along the margin of the thorax ; while the other, under the 

 name of superior epigastric, continues onwards to the abdomen in the 

 original direction of the trunk. The internal mammary artery is 

 covered at its origin by the internal jugular and subclavian veins, 

 and is crossed in front by the phrenic nerve. In the chest it has the 

 costal cartilages and the internal intercostal muscles in front, and lies at 

 first upon the pleura ; but lower down it is separated from the pleura by 

 the triangularis sterni muscle. This artery has two companion veins, 

 which are united into a single trunk at the upper part of the chest. 



Branches.— The branches of this artery are numerous, and are distributed 

 chiefly to the walls of the chest and abdomen. 



(a} The superior phrenic or comes nervi phrenici. a very slender but long branch, 

 arises high in the chest, and descends with the phrenic nei'\^e between the 

 pleura and the pericardium, to the diaphragm, in which it is distributed, anasto- 

 mosing with offsets from the musculo-phrenic and with the inferior phrenic 

 arteries from the abdominal aorta. 



(/y) The mediastinal or thymic branches, of very small size, ramify in the loose 

 connective tissue of the mediastinal space, and supply the thymus body or its 

 remains. Pericardiac branches are given off directly to the upper part of the 

 pericardium, the lower part of which receives some from the musculo-phrenic 

 division. Branches named sternal are also supplied to the triangularis sterni 

 muscle, and to both surfaces of the sternum. 



(<■) Tlie anterior intercostal arteries, two in each space, arise from the internal 

 mammary, either separately, or by a trunk common to the two, which soon 

 divides. The arteries pass outwards, at first between the pleura and the internal 

 intercostal muscles, and aftei^wards between the two layers of intercostals ; they 

 lie, one near the upper and one near the lower rib, in each of the upper five or 

 six intercostal spaces, and inosculate with the corresponding intercostal branches 

 derived from the aortic intercostals. These branches supply the intercostal and 

 pectoral muscles, and give some offsets to the mamma and integument. 



(^Z) The anterior or perforating branches pass forwards from the internal 

 mammary artery through from four to six intercostal spaces, and turning out- 

 wards ramify partly in the pectoralis major, and partly in the integument on the 

 front of the chest. Those placed nearest to the mammary gland supply that 

 organ, and in the female they are of comparatively large size, especially duiing 

 lactation. Some offsets ramify on the sternum. 



(c) The musculo-phrenic artery, the outer of the two branches into which the 

 internal mammary artery divides, inclines downwards and outwards behind the 

 cartilages of the false ribs, perforating the attachment of the diaphragm at the 

 eighth or ninth rib, and becoming gradually reduced in size as it reaches the last 

 intercostal space. It gives branches backwards into the diaphragm ; others, 

 which pass outwards to fonn the anterior intercostals of each space, and are 

 disposed precisely like those which are derived higher up from the main internal 

 mammary ; and some which descend into the abdominal muscles. 



(/) The abdominal branch ov superior epigastric artery of the internal mam- 

 mary, descending into the wall of the abdomen, lies behind the rectus, between 

 the muscle and its sheath ; and afterwards, entering the muscle, terminates in 

 its substance, at the same time anastomosing with the epigastric artery. It also 

 supplies twigs to the broad muscles of the belly, to the skin, and to the dia- 

 phragm ; and one runs forwards upon the side and front of the xiphoid cartilage, 

 where it anastomoses with that of the opposite side. 



Varieties. —The internal mammaiy is occasionally found connected at its 

 origin with the thyroid axis, or with the scapular arteries — these being detached 

 from the thiyroid. It occasionally springs from the second or third part of the 

 subclavian artery (the latter being the more frequent position of the two). In 

 very rare instances it has been found arising from the axillary, the innominate, 

 or the aorta. 



