792 HUMAN ANATOMY. 



Variations. — The passage of the thoracic aorta down the right side of the vertebral cokinin 

 in the upper part of its course and the origin from it of the right subclavian artery have already 

 been discussed in connection with the variations of the aortic arch (page 724). It was there 

 pointed out that both these abnormalities depend upon the more or less perfect persistence of the 

 lower portion of the right primitive aortic arch. Not infrequently a modification of this condi- 

 tion is to be seen in the existence of a small branch arising from the upper part of the thoracic 

 aorta and passing obliquely upward and to the right behind the oesophagus. This is the arteria 

 aberrans, and it is to be regarded as a persistence in a rudimentary condition of the distal por- 

 tion of the right primitive aortic arch. It is regarded by some authors as a normal branch of the 

 thoracic aorta, but it is somewhat inconstant in its occurrence. Occasionally it anastomoses 

 with the first or second intercostal branches of the superior intercostal artery (page 765). 



1. The Bronchial Arteries. — The bronchial arteries (aa. bronchiales) (Fig. 

 718) are somewhat variable in number; while three are usually described, they 

 may be reduced to two or increased to four. They arise from the upper portion 

 of the thoracic aorta and pass to the right and left bronchi, and are continued along 

 these to supply the tissue of the lungs. The right bronchial artery, which very 

 frequently arises from the first right aortic intercostal, passes to the right in front 

 of the oesophagus and applies itself to the posterior surface of the right bronchus, 

 along which it passes to the lung. In its course it "gives off minute branches to 

 the oesophagus, bronchus, and pericardium, and to the lymphatic nodes in its neigh- 

 borhood. 



The left bronchial arteries, which are usually two in number, apply them- 

 selves at once to the posterior surface of the left bronchus as it passes in front of the 

 aorta and are continued along this to the lung. They give off small branches to the 

 oesophagus and to neighboring lymphatic nodes. The upper of the two vessels fre- 

 quently arises by a common stem with the right bronchial, and may be the only one 

 that is present. 



2. The CEsophageal Arteries. — The oesophage^ branches (aa. oesophageae) 

 (/'^'"ig. 718) of the thoracic aorta are also variable in number, forming a series of four or 

 sometimes five or six small vessels which arise in succession from above downward 

 from the anterior surface of the aorta. After a short but somewhat tortuous course, 

 they reach the oesophagus, in the wall of which they branch to form a net-work which 

 receives branches from the bronchial arteries, from the inferior thyroid above and the 

 gastric artery below. 



3. The Mediastinal Arteries. — The mediastinal arteries (rami pericardiac!) 

 are a number of small vessels which arise from the anterior surface of the thoracic 

 aorta and are distributed to the mediastinal lymph-nodes and the posterior surface of 

 the pericardium. 



4. The Aortic Intercostal Arteries. — The aortic intercostals (aa. inter- 

 costales) (Fig. 718) supplying the tissues of the lower intercostal spaces, are usually 

 nine in number on each side, while a tenth, sometimes termed the subcostal artery, 

 runs along the lower border of the last rib, supplying the upper part of the abdom- 

 inal wall. The arteries arise in pairs from the posterior surface of the thoracic aorta 

 and pass outward over the bodies of the vertebrae to the intercostal spaces, those of 

 the right side being, for the most part, somewhat longer than those of the left, 

 owing to the position of the thoracic aorta to the left of the vertebral column through- 

 out the greater portion of its length. Arrived at the intercostal space, each artery 

 passes obliquely outward and upward across the space towards the angle of the rib 

 next above, resting upon the internal intercostal fascia, and covered by pleura. It 

 then pierces the intercostal fascia and, as far as the angle of the rib, runs between the 

 fascia and the external intercostal muscle. On reaching the angle of the rib the artery 

 passes beneath the internal intercostal muscle and is continued around the thoracic 

 wall in the subcostal groove of the rib, and between the two intercostal muscles, to 

 terminate usually by inosculating in front with the upper of the two anterior inter- 

 costal arteries given of? by the internal mammary or the musculo-phrenic to each 

 intercostal space. The arteries which pass to the tenth and eleventh intercostal 

 spaces continue onward beyond the extremities of their corresponding ribs, and, 

 passing between the oblique muscles of the abdomen, anastomose with the deep 

 epigastric artery. The same arrangement occurs in the case of each of the tenth 

 aortic intercostal (subcostal) arteries. These, however, throughout that portion of 



