The lateral sacral arteries are branches of the posterior division of the internal iliac artery. There are usually two of them, a superior and an inferior. The superior lateral sacral artery passes downward and medially. It anastomoses with branches of the middle sacral arteries, to the first anterior sacral foramen which it enters to supply the cauda equina and spinal membranes, and emerges on the back at the upper posterior sacral foramen, to supply the skin and muscles over the sacrum, anastomosing in that locality with the superior gluteal artery. The inferior lateral sacral artery passes medially across the first sacral nerve and the pyriformis muscle. It then runs downward upon the front of the sacrum along the medial side of the anterior sacral foramina and lateral to the sacral sympathetic ganglia. In front of the coccyx it anastomoses with branches of the middle sacral and the lateral sacral arteries of the opposite side. It gives off larger branches which enter the anterior sacral foramina and anastomose with the lateral and anterior spinal arteries. They then emerge from the posterior sacral foramina to be distributed to the skin and muscles over the sacrum. They anastomose with branches from the superior gluteal artery. The lateral sacral arteries supply branches to the pyriformis and coccygeus muscles, to the sacral plexus, and to the rectum, and larger branches to the contents of the spinal canal. Medial branches anastomose with the middle sacral artery.