550 THE ARTERIES 



along which it runs between the intercostal muscles to anastomose with the inferior 

 division of the anterior intercostal branch of the internal mammary artery. It is 

 much smaller than the main intercostal artery, and heli)s to supply the structures 

 in the intercostal space and neighbouring parts. 



(c) The pleural branches ramify beneath the pleura, forming a plexus by 

 anastomosing with like branches above and below. 



(ri) The muscular branches supply the intercostals, serratus magnus, and 

 pectoralis major and minor, and anastomose Avith the long and short thoracic 

 branches of the axillary artery. 



(e) The lateral cutaneous branches run with the lateral cutaneous branches 

 of the intercostal nerves to tlic skin. 



(/) The mammary glandular branches are given off from the intercostal 

 arteries in the third, fourth, and fifth intercostal spaces, and supply the mannnary 

 gland. They are of large size during lactation, and generally require a ligature in 

 the removal of the breast. 



(2) The subcostal artery, or the tAvelfth dorsal as it is sometimes called, 

 follows the same course as the intercostals as far as the head of the twelfth rib. It 

 then passes in compan}^ with the twelfth dorsal nerve along the lower border of the 

 twelfth rib, lying in front of the quadratus lumborum muscle, behind the fascia 

 transversalis. Crossing in front of it is the thickened upper bridge-like margin of 

 this fascia, which stretches across the quadratus and gives origin to some of the 

 fibres of the diaphragm, and is known as the ligamentum arcuatum externum. 

 The subcostal artery anastomoses with the lumliar arteries and external circumflex 

 iliac artery. At the outer edge of the quadratus lumliorum it passes between the 

 abdominal muscles, and is distributed in a manner similar to that of the lumlmr 

 arteries. 



(3) The diaphragmatic branches are small twigs coming off from the thoracic 

 aorta immediately above the diaphragm. They are distributed to the vertebral 

 portion of the diaphragm on its upper surface. 



(4) The aberrans artery is a small twig which, arising from the thoracic aorta 

 near the right bronchial artery, passes upwards and to the right behind the 

 oesophagus and trachea, and is occasionally found to anastomose on the a?sophagus 

 with the arteria aberrans of the superior intercostal artery. It is regarded as the 

 remains of the right aortic dorsal stem (fig. 345). 



THE ABDOMINAL AORTA 



The abdominal aorta (fig. 363), the continuation of the descending or thoracic 

 aorta, begins at the aortic opening in the diaphragm opposite the lower border of 

 the twelfth thoracic vertebra, and ends opposite the middle of the body of the 

 fourth lumbar vertebra on its left side, by dividing into the right and left common 

 iliac arteries. It is at first centrally placed between the pillars of the diaphragm, 

 but as it descends in front of the lumbar vertebrae it leaves the middle line, and, at 

 its bifurcation, lies a little to the left side of the spine. The spot at which the 

 aorta bifurcates is, for all practical purposes, roughly indicated on the surface of 

 the abdomen by a point about half an inch below and a little to the left of the um- 

 bilicus. But the level of its bifurcation may be more accurately determined by a 

 line drawn across the front of the abdomen from the highest point of one iliac crest 

 to the highest point of the other. 



The vena cava inferior, which accompanies the abdominal aorta, lies to its right 

 side. Below, the vein is in contact with the artery and on a somewhat posterior 

 plane; but above, it is separated from the aorta by the right crus of the dia])hragm, 

 and in consequence of the caval opening in the diaphragm being placed further 

 forward than the opening for the aorta, is on an anterior plane. 



Relations. — In front (fig. 364), the aorta is successively crossed from above 

 downwards by the right lobe of the liver, the solar plexus, the lesser omentum, 

 the termination of the oesophagus in the stomach, the ascending layer of the trans- 

 verse meso-colon, the splenic vein or commencement of the vena porta, the pan- 

 creas, the left renal vein, the third portion of the duodenum, the mesentery, the 



