BRANCHES OF THE THORACIC AORTA 565 



of the wall of the hoof, anastomosing above with the circumflex artery of the cor- 

 onary cushion and below with the circumflex artery of the third phalanx. Before 

 passing through the wing it detaches a retrograde branch to the plantar cushion, 

 and after emerging one which ramifies on the outer aspect of the lateral cartilage. 



The terminal part of the digital artery after giving off the preceding vessel is sometimes 

 termed the plantar. 



BRANCHES OF THE THORACIC AORTA 



In addition to the coronary arteries and the common l)rachiocephalic trunk 

 (which have been described), the thoracic part of the aorta gives off branches to the 

 thoracic walls and viscera and to the spinal cord and its membranes. The visceral 

 branches (Rami viscerales) are the bronchial and oesophageal, which arise bj^ a 

 broncho-(esoi)hageal trunk. The parietal branches (Rami parietales) are the 

 intercostal and phrenic arteries. 



1. The broncho-oesophageal (Truncus broncho-oesophageus) is a short, usually 

 bulbous, trunk which arises at the sixth thoracic vertebra from the aorta or in 

 common with the first aortic intercostal arteries. It j^asses (vmder cover of the 

 vena azygos) down the right face of the aorta toward the bifurcation of the tra- 

 chea and divides into l)r()nchial and oesophageal branches. 



(a) The bronchial artery (A. bronchialis) crosses the left face of the oesophagus 

 to the bifurcation of the trachea, where it divides into right and left branches. 

 Each enters the hilus of the corresponding lung above the bronchus, which it 

 accompanies in its ramification. It stipplies the lung tissue and also detaches 

 twigs to the bronchial lymph glands and the mediastinum. 



(6) The cesophageal artery (A. oesophagea) (Fig. 429) is a small vessel which 

 passes backward al)ove the opso]:)hagus in the posterior mediastinum and anasto- 

 moses with the (rsophageal branch of the gastric artery. It detaches twigs to the 

 oesophagus and the mediastinal lymph glands and pleura, and also gives off two 

 branches which pass between the layers of the ligaments of the lungs and ramify 

 in the subpleural tissue. Very commonly there is another arterj^ which runs back- 

 ward ventral to the oesophagus. 



In some cases there is no broncho-oesophageal trunk, the bronchial and oesophageal arising 

 separately. In other cases the second aortic intercostal arises in common with them also. 



2. The intercostal arteries (Aa. intercostales) (Figs. 428, 429) number eighteen 

 pairs. The first arises from the deep cervical artery, the next three from the 

 subcostal branch of the dorsal artery, and the remainder from the aorta. The aortic 

 intercostals arise from the dorsal face of the aorta in pairs close together; the fifth 

 and sixth usually spring from a common stem. Each passes across the body of a 

 vertebra to the corresponding intercostal space, detaches twigs to the vertebra? and 

 the pleura, and divides into dorsal and ventral branches. The dorsal branch (Ramus 

 dorsalis) gives off a spinal branch (Ramus spinalis) which passes through the inter- 

 vertebral foramen, gives twigs to the meml^ranes of the spinal cord, perforates the 

 dura, and reinforces the ventral spinal artery. A muscular branch passes to the 

 muscles and skin of the back. The ventral branch (Ramus ventralis) is much the 

 larger. It passes downward, at first almost in the middle of the intercostal space 

 between the intercostal muscles, then gains the posterior border of the rib and is 

 subpleural. Each is accompanied by a vein and nerve, the artery being in the 

 middle and the vein in front. At the lower part of the space it unites with a ventral 

 intercostal branch of the internal thoracic or the asternal artery. It supplies the 

 intercostal muscles, the ribs and the pleura, and gives off perforating branches 

 which pass out to the serratus magnus, the abdominal muscles, and the skin. 



3. The phrenic arteries (Aa. phrenicte) are two or three small vessels which 

 arise at the hiatus aorticus from the ventral aspect of the aorta, often by a common 



