The 'phrenic arteries [a. phrenicas inferiores], are two small vessels which arise from the aorta, on a level with the under surface of the diaphragm. These little arteries are very irregular in their origin. Supposing them to arise separately one from the other (which will be presently seen to be by no means a constant arrangement), most commonly one is derived from the coeliac artery close to its origin, and the other from the aorta on a level with the under surface of the diaphragm. They soon diverge from each other, and passing across the crura of the diaphragm, incline upwards and outwards upon its under surface.
The artery of the left side, having passed behind the oesophagus, ascends on the left of the oesophageal opening of the diaphragm ; whilst the right phrenic artery, afier having passed behind the liver and the vena cava, lies to the right side of the opening in the diaphragm, which transmits that great vein. Before reaching the central tendon of the diaphragm, each of the arteries divides into two branches, of which one runs forwards towards the anterior margin of the thorax, distributing branches to the diaphragm, and finally anastomosing with the musculo-phrenic branch of the internal mammary artery. The other pursues a transverse direction towards the side of the thorax, and communicates with the terminations of the intercostal arteries.
Each phrenic artery gives small branches (superior capsular) to the suprarenal capsule of its own side ; the left artery sends some branches to the oesophagus, whilst the artery of the right side gives small vessels, which reach the termination of the vena cava. Small offsets descend to the liver between the layers of the peritoneum.
The phrenic arteries are found to vary greatly in their mode of origin, but these deviations seem to have little influence on their course and distribution. In the first place, they may arise either separately, or by a common trunk ; ami it would appear that the latter mode of origin is nearly as frequent as the former. When the two arteries are joined at their origin, the common trunk arises mosl frequently from the aorta; though, sometimes, it springs from the celiac axis. When arising separately, the phrenic arteries are given off sometimes from the aorta, more "frequently from the coeliac axis, and occasionally from the renal; but it most commonly happens that the artery of the right side is derived from one, and that of the left side from another of these sources.* An additional phrenic artery (derived from the left hepatic) has been once met with.
from Quain's anatomy