374 SUBCLAVIAX ARTERIES. 



represents tlie remaining and larger portion of the superficial cervical artery, while 

 the other forms the posterior scapular. 



The transverse cervical artery is sometimes derived directly from the subclavian, 

 beneath or even beyond the scalenus anticus muscle. The transverse cervical some- 

 times gives off the ascending cervical artery. 



When the superficial cervical is separated from the posterior scapular, it sometimes 

 arises from other sources than the thyroid axis, as from the suprascapular or the 

 subclavian artery. 



III. INTERNAL MAMMARY ARTERY. 



The internal mammary artery, remarkable for its length and the number 

 of its branches, arises from the under side of the subclavian, opposite the 

 thyroid axis. It runs forwards and downwards behind the clavicle to the 

 inner surface of the cartilage of the first rib, lying between this and the sac 

 of the pleura : from this point it inclines a little inwards, and then descends 

 vertically behind the costal cartilages, a short distance from the border of 

 the sternum, as far as to the interval between the sixth and seventh 

 cartilages, where it ends by dividing into two branches. One of the 

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

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

 abdominal or superior epigastric, continues onwards to the abdomen in the 

 original direction of the trunk. 



Covered at its origin by the internal jugular vein, like the other large 

 branches of the subclavian artery, the internal mammary soon passes behind 

 the subclavian vein, and is crossed in front by the phrenic nerve which lies 

 between the vein and the artery. 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 plirenici, a very slender but long branch, 

 arises high in the chest, and descends with the phrenic nerve, between the pleura and 

 the pericardium, to the diaphragm, in which it is distributed, anastomosing with 

 offsets from the musculo-phrenic and with the inferior phrenic arteries from the ab- 

 dominal aorta. 



(/>) The mediastinal or tliymic branches, of very small size, ramify in the loose con- 

 nective tissue of the mediastinal space, and supply the remains of the thymus body, 

 which, when in full development, receives its principal branches from the internal 

 mammary artery. 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. 



(c) The 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 afterwards 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. 



(d) The anterior or perforating branches pass forwards from the internal mammary 

 artery through from four to six intercostal spaces, and turning outwards ramify 

 partly in the pectoralis major, and partly in the integument on the front of the chest. 



