THE THORACIC AORTA. 791 



nates by anastomosing with the corresponding digital artery from the superficial palmar 

 arch just before the digital divides into its two terminal branches. Immediately at 

 its origin each palmar interosseous gives off a perforating branch (ramus perforans) 

 which passes dorsally between the adjacent metacarpals to communicate directly with 

 the corresponding dorsal interosseous artery. 



Variations.— The palmar interosseous arteries vary considerably in size, according as the 

 digital branches from the superficial pahnar arch are well or poorly developed (page 784). 

 When the ulnar palmar digital is small, an extra branch may arise from the deep palmar arch, 

 passing along the ulnar border of the little finger. 



9. The Palmar Recurrent Arteries. — The palmar recurrent arteries (Fig. 

 717) are two or three small branches which arise from the concave surface of the deep 

 palmar arch and pass proximally over the carpus to anastomose with the terminal 

 branches of the anterior interosseous and of the anterior radial and ulnar carpal 

 arteries. By the anastomosis of these various arteries there is formed upon the 

 anterior surface of the carpus a net- work, the rete carpale volar e, from which branches 

 are distributed to the wrist and to the carpal articulations. 



The Collateral Circulation in the Forearm. — The brachial artery, after 

 being ligated, will convey blood to the forearm arteries by means of its superior and 

 mferior profunda branches and by the anastomotica magna, which form a rich anas- 

 tomosis at the elbow-joint with the radial recurrent, the anterior and posterior ulnar 

 recurrent, and the posterior interosseous recurrent. The collateral circulation in the 

 parts supplied by the ulnar and radial arteries, after ligation of one or other of these 

 vessels, will be carried on by means of the direct anastomoses between the two 

 arteries in the superficial and deep palmar arches and also by way of the anterior and 

 posterior carpal net-works. To the former of these net-works the radial artery sends 

 contributions from its posterior carpal branch and the ulnar from its posterior carpal 

 and anterior and posterior interosseous branches, while to the latter the radial sends 

 its anterior carpal branch and the ulnar its anterior carpal and anterior interosseous 

 branches. 



THE THORACIC AORTA. 



The thoracic aorta (aorta thoracalis) (Fig. 718) is the continuation of the 

 descending limb of the aortic arch, and begins upon the left side of the body 

 of the fourth thoracic vertebra. It passes downward through the thorax in the 

 posterior mediastinum and terminates below at the diaphragm, behind which it 

 passes to become continuous with the abdominal aorta. In the upper part of its 

 course it lies a litde to the left of the median line, but it tends slightly to the right 

 as it descends, and eventually occupies the median line just before it reaches the 

 diaphragm. 



Relations. — Aiiteriorly it is in relation with the left bronchus and the root of 

 the left lung in its upper part, and it is crossed very obliquely by the oesophagus, 

 which separates it from the pericardium and the posterior surface of the left auricle 

 of the heart. Posteriorly it rests upon the bodies of the eight lower thoracic ver- 

 tebrae, or rather throughout the greater part of its extent upon the anterior common 

 ligament of the thoracic vertebrae, and at about the level of the fifth vertebra has 

 passing obliquely "pward behmd it the thoracic duct and, at the level of the eighth 

 vertebra, the vena hemi-azygos. 



Upon the right side are, above, the oesophagus and lower down the right pleura. 

 The thoracic duct passes upward upon its right side and slightly behind it as far as 

 the fifth thoracic vertebra, and the vena azygos also lies upon its right side, but on a 

 plane slightly posterior to it. On the left side are the left lung and pleura above, 

 and below, the oesophagus, while the vena hemi-azygos also lies upon its left side, but 

 on a somewhat posterior plane. 



Branches. — The branches which arise from the thoracic aorta may be divided 

 into two groups, according as they are distributed to the thoracic viscera or to the 

 parietes. The visceral branches are (i) the brojichial, (2) the oesophageal, and (3) 

 the mediastinal . The parietal branches are (4) the aortic intercostal arteries, and 

 (5) the diaphragmatic branches. 



