The sacrum is composed of several sacral vertebrae (usually 3 or 5) which have fused together to hold up the pelvis. It helps to form the sacroiliac joint with the pelvis and can be seen here at the end of the cat vertebrae (rightmost side) portion pictured. The picture below that is of a sacrum close up, with parts named.


cat_lumbar-sacral.jpgsacrum_and_parts.jpg