THE THORACIC AORTA 601 



passes downward in front of them, and the splanchnic nerves also descend in front 

 by the lower arteries. Each artery then divides into an anterior and a posterior 

 ramus. 



The Anterior Ramus crosses the corresponding intercostal space obliquely toward 

 the angle of the upper rib, and thence is continued forward in the costal groove. It 

 is placed at first between the pleura arid the posterior intercostal membrane, then 

 it pierces this membrane, and lies between it and the Intercostalis externus as far as 

 the angle of the rib; from this onward it runs between the Intercostales externus 

 and internus, and anastomoses in front with the intercostal branch of the internal 

 mammary or musculophrenic. Each artery is accompanied by a vein and a nerve, 

 the former being above and the latter below the artery, except in the upper spaces, 

 where the nerve is at first above the artery. The first aortic intercostal artery 

 anastomoses with the intercostal branch of the costocervical trunk, and may form 

 the chief supply of the second intercostal space. The low r er two intercostal arteries 

 are continued anteriorly from the intercostal spaces into the abdominal wall, and 

 anastomose with the subcostal, superior epigastric, and lumbar arteries. 



Branches. The anterior rami give off the following branches: 



Collateral Intercostal. Lateral Cutaneous. 



Muscular. Mammary. 



The collateral intercostal branch comes off from the intercostal artery near the 

 angle of the rib, and descends to the upper border of the rib below, along which it 

 courses to anastomose with the intercostal branch of the internal mammary. 



Muscular branches are given to the Intercostales and Pectorales and to the 

 Serratus anterior; they anastomose with the highest and lateral thoracic branches 

 of the axillary artery. 



The lateral cutaneous branches accompany the lateral cutaneous branches of the 

 thoracic nerves. 



Mammary branches are given on: by the vessels in the third, fourth, and fifth 

 spaces. They supply the mamma, and increase considerably in size during the 

 period of lactation. 



The Posterior Ramus runs backward through a space which is bounded above 

 and below by the necks of the ribs, medially by the body of a vertebra, and laterally 

 by an anterior costotransverse ligament. It gives off a spinal branch which enters 

 the vertebral canal through the intervertebral foramen and is distributed to the 

 medulla spinalis and its membranes and the vertebra?. It then courses over 

 the transverse process with the posterior division of the thoracic nerve, supplies 

 branches to the muscles of the back and cutaneous branches which accompany 

 the corresponding cutaneous branches of the posterior division of the nerve. 



The subcostal arteries, so named because they lie below the last ribs, constitute 

 the lowest pair of branches derived from the thoracic aorta, and are in series with 

 the intercostal arteries. Each passes along the lower border of the twelfth rib 

 behind the kidney and in front of the Quadratus lumborum muscle, and is accom- 

 panied by the twelfth thoracic nerve. It then pierces the posterior aponeurosis 

 of the Trans versus abdominis, and, passing forward between this muscle and the 

 Obliquus internus, anastomoses with the superior epigastric, lower intercostal, and 

 lumbar arteries. Each subcostal artery gives off a posterior branch which has a 

 similar distribution to the posterior ramus of an intercostal artery. 



The superior phrenic branches are small and arise from the lower part of the 

 thoracic aorta; they are distributed to the posterior part of the upper surface of 

 the diaphragm, and anastomose with the musculophr,enic and pericardiacophrenic 

 arteries. 



A small aberrant artery is sometimes found arising from the right side of the tho- 

 racic aorta near the origin of the right bronchial. It passes upward and to the right 



