THE THORACIC AORTA. 793 



their course in which they are in relation to the twelfth ribs, rest upon the quadratus 

 lumborum muscles, beneath the transversalis fascia, and at the outer border of that 

 muscle pass beneath the fibres of the transversalis abdominis, and, more laterally, 

 perforating the internal oblique, come to lie between that muscle and the external 

 oblique. 



Relations. — In the first portion of their course, while passing over the bodies 

 of the vertebree, the right aortic intercostals are crossed by the thoracic duct and by 

 the vena azygos, and the upper ones are also crossed by the oesophagus. Those 

 of the left side are crossed by the vena hemiazygos, and both sets are covered 

 by the pleura. Opposite the heads of the ribs they are crossed by the ganglionated 

 cord of the sympathetic nervous system, the lower ones also by the splanchnic nerves, 

 and in their course through the intercostal spaces they are in relation to the inter- 

 costal veins and nerves, each artery lying below its corresponding vein and above the 

 nerve, but on a plane slightly posterior to both. The arteries of the upper spaces lie 

 at first below the corresponding nerves, but as they approach the lower borders of 

 their ribs they cross the nerves obliquely, and throughout the greater part of their 

 course possess the relation described. 



Branches. — Each artery gives off small branches to the bodies of the vertebrje and to the 

 pleura, and throughout its course through the intercostal space numerous, 



(a) Muscular branches, which supply the intercostal muscles, the serratus magnus, and 

 the pectorales major and minor, anastomosing with the thoracic branches from the axillary 

 artery. The vessels of the lower spaces and the subcostal also supply the upper portions of 

 the abdominal muscles, the subcostal anastomosing with branches of the uppermost lumbar 

 artery and with the ascending branch of the superficial circumflex iliac ; the lower vessels also 

 give off numerous branches to the diaphragm which anastomose with the phrenic arteries 

 from the abdominal aorta. Some of the muscular branches which arise from the vessels of the 

 third, fourth, and fifth spaces send branches to the mammary gland, assisting the perforating 

 branches of the internal mammary and the long thoracic branch of the axillary in the supply of 

 that structure. These intercostal mammary branches (rami mammarii laterales) may become 

 greatly enlarged during lactation, and may give rise to considerable hemorrhage in the operation 

 for removal of the gland. 



In addition, each aortic intercostal gives off a dorsal, a lateral cutaneous, and a collateral 

 branch. 



(6) The dorsal branch (ramus posterior) arises from each artery, just as it enters its inter- 

 costal space, and passes directly backward, in company with the posterior division of the 

 corresponding spinal nerve, between the necks of the adjacent ribs and internal to the superior 

 costo-transverse ligament. Having reached the vertebral groove, it divides into a spina/ and a 

 muscular branch. The former (ramus spinalis) passes through the intervertebral foramen in 

 company with the root of the spinal nerve, and, within the spinal canal, gives off branches to 

 the body of the vertebra and its neural arches and to the dura mater, and also branches which 

 pass to the spinal cord and anastomose with the anterior and posterior spinal arteries. The 

 muscular branch (ramus muscularis) continues posteriorly in the direction of the main stem of 

 the vessel and divides into an external and an internal branch which pass between the principal 

 masses of the dorsal musculature, supplying these and terminating in branches to the integu- 

 ment of the back. 



(r) The lateral cutaneous branch (ramus cutaneus lateralis) arises at about the axillar>' line 

 and perforates the external intercostal muscle in company with the lateral cutaneous branch of 

 the corresponding intercostal nerve. It is distributed with the ner\'e to the integument of the 

 lateral portions of the thorax, also supplying the serratus magnus and the pectoral muscles 

 and anastomosing with the perforating branches of the internal mammary and with the thoracic 

 branches of the axillary arten.-. 



{d ) The collateral branch arises as the intercostal approaches the angle of its rib. It 

 passes obliquely outward and downward to the upper border of the rib next below, along which 

 it runs to terminate by anastomosing with the lower of the two anterior intercostal branches 

 given off by the internal mammary or the musculo-phrenic to each intercostal space. The 

 collateral branches of the three lower intercostals are small and inconstant and, when present, 

 terminate in the abdominal wall. 



Variations.— The intercostal arteries of the first and second spaces usually arise from the 

 superior intercostal branch of the subclavian, but occasionally the arter^^ of the second space, 

 and more rarely that of the first, may arise from the thoracic aorta. Or, conversely, the arteries 

 of the third and fourth intercostal spaces, as well as those of the first and second, may arise 

 from the superior intercostal, the aortic intercostals being correspondingly reduced in number. 



