12 THORAX 



muscle of the abdomen. The anterior continuations of several 

 of the intercostal nerves give additional muscular twigs to 

 the adjacent muscles. The fourth, fifth, and sixth supply the 

 transversus thoracis, and the lower six supply the internal 

 oblique, the transversus abdominis, and the rectus abdominis 

 muscles. 



Dissection. If the dissector has not been successful in dis- 

 playing the intercostal vessels in the spaces in which he has 

 dissected the nerves an attempt should be made to dissect out 

 the vessels in a fresh space, but a satisfactory demonstration of 

 the vessels in the anterior parts of the spaces will not be possible 

 unless the subject has been well injected. 



Arteriae Intercostales. In each intercostal space one artery 

 is found passing dorso-ventrally ; and in each of the upper 

 nine intercostal spaces two anterior intercostal arteries run 

 ventro-dor sally. 



In the uppermost 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 

 are branches of the internal mammary artery, whilst those of 

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

 phrenic artery. There are no anterior intercostal arteries in 

 the last two spaces. 



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, 

 at a higher level than the corresponding nerves and below 

 the accompanying vein. Midway between the vertebral 

 column and the sternum each aortic intercostal artery divides 

 into two branches, which pass forwards in relation to the 

 upper and lower margins of the intercostal space, and either 

 the trunk or the upper branch gives off a twig which accom- 

 panies the lateral cutaneous nerve. 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 upper aortic intercostal 

 vessels. 



