THORACIC WALL 7 



In the upper two spaces the vessels which run dorso- 

 ventrally are derived from the superior intercostal division of the 

 costo-cervical branch of the subclavian artery; in the lower nine 

 spaces they spring directly from the aorta, and are called the 

 aortic intercostal arteries. 



The anterior intercostal arteries of the upper six spaces 

 proceed directly from the internal mammary, whilst those of 

 seventh, eighth, and ninth spaces arise from the musculo- 

 phrenic artery. 



The intercostal vessels are distributed for the most part 

 between the two muscular strata. From the angles of the 

 ribs onwards to a point midway between the vertebral column 

 and sternum, the aortic intercostal arteries lie under shelter of 

 the lower margins of the ribs which bound the spaces superiorly, 

 and at a higher level than the corresponding nerves. Then 

 each divides into two branches, which pass ventrally in 

 relation to the upper and lower margins of the intercostal 

 space. They give off small branches which accompany the 

 lateral cutaneous nerves. The lower two aortic intercostal 

 arteries are carried onwards into the abdominal wall. The 

 branches of the superior intercostal artery are disposed in a 

 manner similar to the aortic intercostal vessels. 



The anterior intercostal arteries are two in number for each 

 space, except the last two. At their origins they lie under 

 cover of the internal intercostal muscles, and they run later- 

 ally in relation to the upper and lower margins of the ribs 

 bounding the spaces. After a short course they pierce the 

 internal intercostal muscles, and end by anastomosing with 

 the aortic and superior intercostal arteries. 



Dissection. The dissector should next proceed to remove the intercostal 

 muscles. This dissection must be done with great care, for immediately 

 subjacent to the internal intercostals and the ribs is the delicate pleural 

 membrane which lines the inner surface of the chest wall. The membrane 

 must not be injured or detached from the deep surfaces of the ribs during 

 this stage of the dissection. As the internal intercostal muscles are removed, 

 the anterior perforating branches of the internal mammary and musculo- 

 phrenic arteries, and the anterior cutaneous nerves must be preserved. 



When the muscles are removed the internal mammary artery with its 

 two accompanying veins will be seen behind the costal cartilages, about 

 half an inch from the side of the sternum. Clean these arteries in the 

 intervals between the cartilages and note the small lymph glands 

 which lie beside them. Each internal mammary artery ends by dividing 

 into superior epigastric and musculo- phrenic terminal branches in the 

 interval between the sixth and seventh rib cartilages. Most likely this 

 space will be so narrow that a view of the bifurcation cannot be obtained. 

 If this is the case, pare away the edges of the cartilages over the artery, or, 

 II 16 



