Sacrum a lateral surface. The base of the sacrum, which is broad and expanded, is directed upward and forward. On either side of the base is a large projection known as an ala of sacrum and these alae (wings) articulate with the sacroiliac joints. The alae support the psoas major muscles and the lumbosacral trunk which connects the lumbar plexus with the sacral plexus. In the articulated pelvis the alae are continuous with the iliac fossa. Each ala is slightly concave from side to side, and convex from the back and gives attachment to a few of the fibers of the
Sacrum moves anteriorly, "counter-nutation". In upright vertebrates, the sacrum is capable of slight independent movement along the sagittal plane. On bending backward the top (base) of the sacrum moves forward relative to the ilium; on bending forward the top moves back. The "sacrum" refers to all of the parts combined. Its parts are called "sacral vertebrae" when referred individually. In some cases the sacrum will consist of six pieces or be reduced in number to four. The bodies of the first and second vertebrae may fail to unite. Sometimes the uppermost transverse tubercles are not joined to the rest of the