456 THE DESCENDING THORACIC AORTA. 



anastomoses with one of the anterior intercostal branches derived from the internal 

 mammary artery. 



The first of the aortic intercostal arteries has an anastomosis with the superior 

 intercostal artery from the subclavian, and not unfrequently sends upwards a con- 

 siderable branch, which supplies the second space wholly or in great part. The last 

 two are prolonged into the abdominal wall, where they communicate with the 

 epigastric artery, and with the lumbar branches of the abdominal aorta. 



Each intercostal artery is accompanied, as it runs outwards between the ribs, by 

 a corresponding vein, and by an intercostal nerve ; the vein being usually above, 

 and the nerve below it. 



Branches. (a) The posterior or dorsal branch of each intercostal artery passes 

 backwards to the inner side of the superior costo-transverse ligaments, along with 

 the posterior branch of the corresponding 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 spinse, 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 ilio-costalis ; some twigs reach the superficial muscles 

 and the integuments. 



The spinal branch is distributed partly to the cord and its membranes, and partly 

 to the bones, in the manner described on p. 422. 



(b) The collateral intercostal branch, long and slender, arises near the angle of the 

 upper rib of the space, inclines downwards, and is continued along the border of the 

 lower rib, to anastomose in front with an anterior intercostal branch of the internal 

 mammary artery. There are thus in each intercostal space two terminal branches 

 of the intercostal artery communicating with branches of the internal mammary. 



Both the main trunk and the collateral branch give offsets to the intercostal 

 muscles and ribs, and small twigs which anastomose beneath the pleura with one 

 another and with branches of the internal mammary and bronchial arteries (see 

 p. 428). Other branches supply the muscles covering the thorax and anastomose 

 with the thoracic branches of the axillary artery, and one accompanies the lateral 

 cutaneous branch of the intercostal nerve to the skin. From the arteries in the 

 third, fourth, and fifth spaces branches are sent to the mammary gland in the female. 



Small anastomotic branches may often be found uniting adjacent intercostal 

 arteries both in front of and behind the necks of the ribs. Similar anastomoses 

 occur between the posterior branches behind the transverse processes. 1 



Varieties. The number of trunks by which the intercostal arteries arise is subject to 

 much variation : two or even three arteries of the same side, especially the upper ones, some- 

 times arise by a single stem. One or two of these vessels may be absent on one side, the 

 corresponding spaces being supplied by branches from the neighbouring intercostal arteries. 



ABDOMINAL AORTA. 



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

 about the lower border of the last dorsal vertebra, and terminates below by dividing 

 into the two common iliac arteries. The bifurcation usually takes place about half- 

 way down the body of the fourth lumbar vertebra and a little to the left of the middle 

 line, a point which is nearly on a level with a line drawn from the highest part of 

 one iliac crest to the other, and which corresponds to a spot on the front of the 

 abdomen slightly below and to the left side of the umbilicus. Its length is about 

 five inches. 



1 W. J. Walsham, Journ. Anat , xvi, 443, 1882. 



