The lumbar arteries, [a. lumbales], are analogous to the intercostal arteries, not only in their mode of origin, direction, and size, but in a great measure in the manner of their distribution also.
Thus, as the intercostal arteries communicate wiih the branches of the internal mammary upon the thorax, so the lumbar arteries, by anastomosing with the epigastric, have a nearly similar relation to the walls of the abdomen. The lumbar arteries arising from the back part of the aorta, are usually four in number on each side. They pass outwards, (each crossing the middle of the body of the corresponding lumbar vertebra,) and soon dip deeply under the psoas muscle, between it and the bodies of the vertebra;. The two upper arteries are likewise under the pillars of the diaphragm ; those on the right side are covered by the vena cava. At the interval between the transverse processes, each lumbar artery divides into a dorsal and an abdominal branch.
The abdominal branch of each lumbar artery runs outwards behind the quadratus lumborum, — the lowest of these branches not infrequently in front of that muscle. Continuing outwards between the abdominal muscles, the artery ramifies within them, and maintains communications with branches of the epigastric and internal mammary in front, with the terminal branches of the intercostals above, and with those of the ilio-lumbar and circumflex iliac arteries below.
The dorsal branch [ramus dorsalis] of each lumbar artery, like the corresponding branch of the intercostal arteries, gives off, immediately after its origin, an offset (named spinal), which enters the spinal canal. The dorsal branch then, proceeding backwards with the posterior division of the corresponding lumbar nerve between the transverse processes of the vertebrae, divides into smaller vessels, which are distributed to the muscles and the integument of the back.
The spinal artery [ramus spinalis] enters the spinal canal through the intervertebral foramen, and, having given an offset which runs along the nerves to the dura mater and cauda equina and communicates with the other spinal arteries, divides into two branches, which are distributed to the bones in the following manner: — one curves upwards on the back part of the body of the vertebra above, near to the root of the pedicle, whilst the other descends in a similar manner on the vertebra below; and each communicates with a corresponding branch from the neighboring spinal artery. As this arrangement prevails on both sides and throughout the whole length of the spine, there is formed a double series of arterial arches behind the bodies of the vertebrae, the convexities of which are turned towards each other. The arches are moreover joined together across the bodies of the vertebrae by transverse branches. From this interlacement of vessels, numerous ramifications are distributed to the periosteum and the bone. The lumbar arteries of opposite sides, instead of taking their origin separately from the aorta, occasionally commence by a common trunk, whose branches pass out laterally, and continue their course in the ordinary way. Two arteries of the same side are sometimes conjoined at their origin. On the last lumbar vertebra, the place of a lumbar artery is often taken by an offset from the middle sacral artery, and the ilio-lumbar compensates for the absence of the lumbar vessel amongst the muscles.
From Quain's anatomy.