422 THE THORACIC AORTA. 



surface of the external intercostal muscle, and in the back of the space 

 is separated from the pleura l)y a fascia only, but further outwards it 

 lies between the two layers of intercostal muscles. Extending forwards 

 in contact with the rib above, it finally anastomoses with one of the 

 anterior intercostal branches derived from the internal mammary 

 artery, and with the thoracic branches of the axillary artery. 



The first of the aortic intercostal arteries has an anastomosis with 

 the superior intercostal proceeding- from the subclavian artery ; and 

 the last three are prolonged into the abdominal muscles, where they 

 communicate with the epigastric artery in front, with the phrenic 

 arteries at the side, and with the lumbar branches of the abdominal 

 aorta lower down. 



Each intercostal artery is accompanied, as it runs outwards between 

 the ribs, by a corresponding vein, and by one of the dorsal nerves ; the 

 vein usually being uppermost, and the artery next below it. 



Branches. — (a) The jfosferio?' a?- dor.sal branch of each intercostal artery passes 

 backwards to the inner side of the anterior costo-transverse ligament, along with 

 the posterior branch of the corresponding spinal nerve ; and, having furnished 

 an offset to the spinal canal, reaches the muscles of the back, and divides into 

 an internal and an external branch. The internal branch is directed towards 

 the spinous processes, on or through the multifidus spinas, and ramifies in the 

 muscles and the skin. The external branch turns outwards under the longissimus 

 dorsi. and is distributed between that muscle and the sacro-lumbalis ; some twigs 

 reach the superficial muscles and the integuments. 



The .'f/i/?«rtZ branches of the aortic intercostal arteries are distributed partly to 

 the cord and its membranes, and partly to the bones, in the same manner as the 

 spinal branches of the lumbar ai-teries, to the description of which the reader is 

 referred. 



(//) The coUnfernl aitcrcostal branch, long and slender, arises near the place 

 wdiere the main trunk comes in contact with the upper rib of the space, and 

 inclinino- downwards approaches the border of the lower rib, suppljdng the l^one 

 and the intercostal muscles, and anastomosing in front with an anterior inter- 

 costal branch of the internal mammary arteiy. There are thus in each inter- 

 costal space two terminal branches of the intercostal arteries communicating vnih. 

 the branches of the internal mamm.aiy. 



Varieties. — The number of the intercostal arteries is subject to much varia- 

 tion : the thii'd, fourth, and fifth vessels may be entirely absent upon one 

 side, the corresponding spaces being supplied from neighbouring arteries. Two 

 arteries frequently arise from the aorta by a single stem. In this way the first 

 lumbar artery is sometimes conjoined with the twelfth intercostal (Henle). 



ABDOMINAL AOHTA. 



The aorta, after having passed the diaphragm, is thus named. It 

 commences on the front of the last dorsal vertebra, and terminates. 



Fig. 275. — View of the Abdominal Aorta and its Pkixcipal Br.ANcnES (from. 



Tiedemann). J 



fl, ensifonn portion of the sternum ; h, vena cava inferior passing through the tendon 

 of the diaphragm ; r, the a\sophagus passing through the muscular portion ; d and c, ten- 

 dinous parts of the riglil and left crura ; /, /', the right and left kidneys with their supni- 

 renal bodies ; n, 'J, the ureters ; h, the upper part of the iirinary bladder ; i, i, the right 

 and left vasa deferentia passing up from tlie bladder to the internal inguinal apertures ; 

 1c, the rectum, divided and tied near its upi^er pai't ; 1, 1, the abdominal aorta ; 1', the- 

 middle sacral artery ; 2, 2', the right, 3, 3', the left inferior phrenic arteries, represented 

 as arising by a short common stem from the front of the aorta immediately below the 

 meeting of the crura of the diaphragm ; 4, the trunk of the cadiac axis ; 5, the superior 

 mesenteric artery ; 6, 6, the renal arteries ; 6', 6', the suprarenal arteries arising partly 



