THE THORAX. 321 



The superior epigastric continues downward, enters the 

 under surface of the rectus muscle (piercing its sheath), and 

 finally terminates in anastomotic branches with the deep 

 epigastric. 



The musculophrenic passes downward and outward 

 across the costal cartilages of the false ribs and along the 

 origin of the diaphragm, which it perforates over the ninth 

 rib, to the tenth or eleventh intercostal space. In its course 

 it supplies the intercostal muscles and the diaphragm. 



Relations. In the neck, the internal mammary is crossed 

 in front by the phrenic nerve, which passes from the outside 

 to the inner side of the artery, and by the subclavian vein. 

 The artery in the upper part of its course is in contact with 

 the pleura, but in the lower is separated from it by the tri- 

 angularis sterni muscle. 



Branches. A small artery to accompany the phrenic 

 nerve, given off at the upper opening of the thorax. 



The anterior intercostal arteries are a pair in each of the 

 upper six intercostal spaces. These turn outward to anas- 

 tomose with the aortic intercostals, supplying the surround- 

 ing parts. 



The perforating. One for each of the six upper spaces. 

 Pass forward and are distributed to the parts upon the front 

 of the chest. The mammary gland receives blood from the 

 second, third, and fourth. 



Besides these there are small branches to the mediasti- 

 num, sternum, and pericardium. 



The internal mammary artery is accompanied by two 

 veins which empty into the innominate vein of that side. 



Parts Within the Thorax. If the pleurae have been saved 

 uninjured in removing the sternum and cartilages (and ribs), 

 they will be seen to form a closed sac within which the 

 lungs are contained. By inflating the lungs they will ex- 



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