THE THORAX 933 



In this manner it enters the sheath of the rectus abdominis, l^'ing 

 at first behind the muscle, but afterwards entering it. In the muscle 

 it anastomoses with the deep epigastric artery, which is a branch of 

 the external iliac. 



The branches of the superior epigastric artery will be found 

 described on p. 663. 



The internal mammary veins {vence comites) are two in number, 

 and lie one on each side of the artery. They are formed, respectively, 

 by the union of the venae comites of the musculo-phrenic and 

 superior epigastric arteries. In their course they receive tributaries 

 corresponding to the branches of the artery. About the level of 

 the first intercostal space the outer vein crosses over the artery and 

 joins the inner vein to form a single vessel, which opens into the 

 corresponding innominate vein. The internal mammary veins are 

 provided with valves at intervals. 



Sternal or Internal Mammary Lymphatic Glands. — These glands 

 form a chain along the course of the internal mammary vessels on 

 either side, there being one, or it may be two, in the anterior ex- 

 tremity of each of the upper six intercostal spaces. 



They receive their afferent vessels from (i) the inner third of the 

 mammary gland, {2) the anterior half of the costal pleura, (3) the 

 anterior halves of the external and internal intercostal muscles, 



(4) the lymphatics which accompany the superior epigastric artery, 

 and (5) the anterior diaphragmatic glands. Their efferent vessels 

 pass to the thoracic duct, or to the rigJit lymphatic duct, according 

 to the side on which the glands lie. 



Triangularis Sterni — Origin. — (i)'The deep surface of the ensiform 

 process and body of the stemiun, close to the lateral border, and 

 extending as high as the level of the third costal cartilage ; and 



(5) the deep surfaces of the lower two or three true costal cartilages 

 at their sternal ends. 



Insertion. — ^The deep surfaces and lower borders of the costal 

 cartilages from the sixth to the second. The insertion takes place 

 by separate slips, and one or two of the upper slips may be partially 

 attached to the rib itself. 



Nerve-supply. — The intercostal nerves of the adjacent spaces. 



The lowest fibres of the muscle are horizontal ; the succeeding 

 fibres pass obliquely upwards and outwards ; and the upper fibres 

 are almost vertical. 



Action. — ^To depress the anterior extremities of the ribs, and so 

 take part in expiration. 



The muscle forms a thin musculo-tendinous sheet, which is 

 situated on the deep surfaces of the costal cartilages and side of the 

 sternum, and it is serially continuous with the transversalis ab- 

 dominis muscle. It supports the internal mammary vessels, sternal 

 glands, and certain of the intercostal nerves, whilst its deep surface 

 rests upon the parietal pleura. 



