546 THE BLOOD-VASCULAR SYSTEM. 



ARTERIES OF THE TRUNK. 

 THE DESCENDING AORTA. 



The Descending Aorta is divided into two portions, the thoracic and abdominal, 

 in correspondence with the two great cavities of the trunk in which it is situated. 



THE THORACIC AORTA. 



The Thoracic Aorta commences at the lower border of the fourth dorsal verte- 

 bra, on the left side, and terminates at the aortic opening in the Diaphragm, in front 

 of the lower border of the last dorsal vertebra. At its commencement it is situated 

 on the left side of the spine ; it approaches the median line as it descends, and at 

 its termination lies directly in front of the column. The direction of this vessel 

 being influenced by the spine, upon which it rests, it describes a curve which is 

 concave forward in the dorsal region. As the branches given off from it are small, 

 the diminution in the size of the vessel is inconsiderable. It is contained in the 

 back part of the posterior mediastinum. 



Relations. It is in relation, in front, from above downward, with the root of 

 the left lung, the pericardium, the oesophagus, and the Diaphragm : behind, with 

 the vertebral column and the vena azygos minor ; on the right side, with the vena 

 azygos major and thoracic duct; on the left side, with the left pleura and lung. 

 The oesophagus, with its accompanying nerves, lies on the right side of the aorta 

 above ; but at the lower part of the thorax it gets in front of the aorta, and close 

 to the Diaphragm is situated to its left side. 



PLAN OF THE RELATIONS OF THE THORACIC AORTA. 



f u front. 



Root of left lung. 

 Pericardium. 

 (Esophagus. 

 Diaphragm. 



Right tide. { \ Left side. 



(Esophagus (above). I Thoracic \ Pleura. 



Vena azygos major. \ Aorta< ) Left lung. 



Thoracic duct. \ J (Esophagus (below). 



Behind. 



Vertebral column. 

 Superior and interior azygos minor veins. 



The aorta is occasionally found to be obliterated at a particular spot viz., at the junction of 

 the arch with the thoracic aorta, just below the ductus arteriosus. Whether this is the result of 

 disease or of congenital malformation is immaterial to our present purpose ; it affords an interest- 

 ing opportunity of observing the resources of the collateral circulation. The course of the anas- 

 tomosing vessels, by which the blood is brought from the upper to the lower part of the artery, 

 will be found well described in an account of two cases in the Pathological Transactions, vols. viii. 

 and x. In the former (p. 162) Mr. Sydney Jones thus sums up the detailed description of the 

 anastomosing vessels : ''The principal communications by which the circulation was carried on, 

 were Firstly, the internal mammary, anastomosing with the intercostal arteries, with the phrenic 

 of the abdominal aorta by means of the musculo-phrenic and comes nervi phrenici, and largely 

 with the deep epigastric. Secondly, the superior intercostal, anastomosing anteriorly by means 

 of a large branch with the first aortic intercostal, and posteriorly with the posterior branch of the 

 same artery. Thirdly, the inferior thyroid, by means of a branch about the size of an ordinary 

 radial, formed a communication with the first aortic intercostal. Fourthly, the transversalis colli, 

 by means of very large communications with the posterior branches of the intercostals. Fifthly, 

 the branches (of the subclavian and axillary) going to the side of the chest were large, and 

 anastomosed freely with the lateral branches of the intercostals." In the second case also (vol. 

 x. p. 97) Mr. Wood describes the anastomoses in a somewhat similar manner, adding the remark 

 that " the blood which was brought into the aorta through the anastomoses of the intercostal 

 arteries appeared to be expended principally in supplying the abdomen and pelvis, while the sup- 

 ply to the lower extremities had passed through the internal mammary and epigastrics. " 



