Of the 13 pairs of intercostal arteries, the first intercostal arises form dorsal artery.
The second to fifth pair form the sub costal artery and the rest from the thoracic aorta.
These aortic intercostal arteries arise in pairs from the dorsal face of the thoracic aorta at regular intervals.
Their origins (of each pair) are close together and they diverge as they pass upwards across the bodies of the dorsal vertebrae and dorsal sympathetic trunk. They cross the vena hemiazygos on the left.
On gaining the upper part of the corresponding intercostal space each intercostal artery gives off branches to the pleura and vertebra and divides into a dorsospinal and an intercostal branch.
The dorsospinal branch divides into spinal and muscular branches, the former to reinforce the inferior spinal artery and the latter to supply the spinal muscles.
The intercostal branch is at first subpleural runs between external and internal muscles, to about the middle of the intercostal space reaches the posterior border of the rib and runs down in company with the nerve and vein to the ventral part.
It furnishes branches in its course to the intercostal muscles and furnishes some branches, which perforate the intercostal space and pass out as perforating intercostal arteries to supply serratus thoracis and abdominal muscles and skin.
The terminal branches of each intercostal branch anastomose with the ascending branches of the internal thoracic and musculophrenic arteries.
Phrenic arteries
Parietal unpaired. There are two or three branches that arise either directly from the thoracic aorta, a little in front of the hiatus aorticus or from the coeliac artery, left ruminal artery, intercostal or lumber arteries and these supply the crura of the diaphragm.
