BRANCHES OF THE THORACIC AORTA. 495 



press upon the thoracic duct; or it may burst into the pleural cavity, or into the trachea or 

 lung ; and lastly, it may open into the posterior mediastinum. 



The aorta is, comparatively often, found to be obliterated at a particular spot, viz., at the 

 1'unction 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 interesting opportunity of observing the resources of the collateral circulation. The 

 course of the anastomosing vessels, by which the blood is brought from the upper to the lower 

 part of the artery, will be fouiid 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 inter- 

 costal 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 poste- 

 riorly, 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 inter- 

 costal. Fourthly, the transversalis colli, by means of very large communications with the poste- 

 rior 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 inter- 

 costals." 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 supply to the lower extremities had passed through 

 the internal mammary and epigastrics." 



BRANCHES OF THE THORACIC AORTA. 



Pericardiac. (Esophageal. 



Bronchial. Posterior mediastinal. 



Intercostal. 



The pericardiac are a few small vessels, irregular in their origin, distributed 

 to the pericardium. 



The bronchial arteries are the nutrient vessels of the lungs, and vary in num- 

 ber, size, and origin. That of the right side arises from the first aortic inter- 

 costal, or by a common trunk with the left bronchial, from the front of the 

 thoracic aorta. Those of the left side, usually two in number, arise from the 

 thoracic aorta, one a little lower than the other. Each vessel is directed to the 

 back part of the corresponding bronchus, along which they run, dividing and 

 subdividing, upon the bronchial tubes, supplying them, the cellular tissue of 

 the lungs, the bronchial glands, and the oesophagus. 



The oesophageal arteries, usually four or five in number, arise from the front of 

 the aorta, and pass obliquely downwards to the oesophagus, forming a chain of 

 anastomoses along that tube, anastomosing with the cesophageal branches of the 

 inferior thyroid arteries above, and with ascending branches from the phrenic 

 and gastric arteries below. 



The posterior mediastinal arteries are numerous small vessels which supply the 

 glands and loose areolar tissue in the mediastinum. 



The intercostal arteries arise from the back part of the aorta. They are usually 

 ten in number on each side, the superior intercostal space (and occasionally the 

 second one) being supplied by the superior intercostal, a branch of the subcla- 

 vian. The right intercostals are longer than the left, on account of the position 

 of the aorta to the left side of the spine ; they pass outwards, across the bodies 

 of the vertebrae, to the intercostal spaces, being covered by the pleura, the 

 oesophagus, thoracic duct, sympathetic nerve, and the vena azygos major ; the 

 left passing beneath the superior intercostal vein, the vena azygos minor, and 

 sympathetic. In the intercostal spaces, each artery divides into two branches, 

 an anterior, or proper intercostal branch ; and a posterior, or dorsal branch. 



The anterior branch passes outwards, at first lying upon the External Inter- 

 costal muscle, covered in front by the pleura, and a thin fascia. It then passes 

 between the two layers of Intercostal muscles, and, having ascended obliquely 

 to the lower border of the rib above, divides, near the angle of that bone, into 



