The sacral vertebrae (see Figure 3.19) begin to fuse with each other at puberty. They are completely fused between 25–30 years of age. The fused sacrum is tri-angular with the base located superiorly and the apex pointing downward. The lateral part of the sacrum has articulating surfaces for the pelvic girdle. Large mus-cles of the thigh are attached to its large surface. The sacrum also protects the lower end of the digestive tract and organs of reproduction and excretion. The upper end of the sacrum articulates with the last lum-bar vertebra. Internally, the sacral canal is a continu-ation of the vertebral canal. The nerves from the spinal cord, along with the membranes, continue along the length of the sacral canal and enter and leave the canal through the foramen in the sacrum.
The coccyx also consists of vertebrae, which begin to fuse by about age 26. It provides attachment to lig-aments and anal muscles.