This section is from the book "Anatomy Of The Arteries Of The Human Body", by John Hatch Power. Also available from Amazon: Anatomy of the Arteries of the Human Body, with the Descriptive Anatomy of the Heart.
The Middle Sacral Artery, usually smaller than the lumbar arteries, arises from the posterior part of the aorta a little above its bifurcation; it then descends on the front of the spine, separated from it by the anterior common ligament; and then on the middle line of the sacrum, separated from it by the periosteum of that bone; it is covered in front by the aorta, left common iliac vein, and by the pelvic viscera. It is separated from the lateral sacral of either side by the corresponding trunk of the sympathetic nerve. Inferiorly it terminates by dividing into two branches, right and left, which communicate, in the form of a double arch, with the right and left lateral sacral arteries. Opposite each bone of the sacrum this artery sends off transverse branches to either side, which supply the periosteum, and communicate with the lateral sacral and haemorrhoidal arteries.