The coccyx is not part of the sacrum; they are two distinct, connected bones located at the base of the spine. The sacrum is a large, triangular bone that sits below the lumbar spine, forming the posterior wall of the pelvis. The coccyx, commonly called the tailbone, is a much smaller bone positioned at the very end of the vertebral column, articulating with the sacrum’s tip. Both structures form from the fusion of smaller vertebrae, but they remain separate entities with different roles in body structure and movement.
The Sacrum: Structure and Role
The sacrum is a large, triangular bone formed by the complete fusion of five individual sacral vertebrae (S1 through S5), a process typically complete in early adulthood. This fusion creates a strong, single structure positioned like a wedge between the two hip bones. It serves as the keystone of the pelvis and acts as a fixed attachment point for the spine.
Its primary function is to distribute the weight of the upper body to the lower limbs via the sacroiliac joints connecting to the iliac bones. This connection is essential for stability when standing, walking, or bearing weight. The sacrum’s shape allows it to absorb and transfer forces from the torso to the legs, providing a stable foundation. The bone also contains the sacral canal, which protects nerve roots supplying the lower body.
The Coccyx: Structure and Role
The coccyx, or tailbone, is the terminal segment of the vertebral column. It is a small, curved bone composed of three to five small vertebrae that are fused or semi-fused together. Unlike the sacrum’s complete fusion, the coccygeal vertebrae may remain partially separate, especially in younger individuals.
The coccyx serves as an anchor point for numerous muscles, ligaments, and tendons that form the pelvic floor. The gluteus maximus and the levator ani muscle, a major component of the pelvic floor, both attach here. This support is important for maintaining the position of the pelvic organs and assisting with defecation. The coccyx also bears a small amount of weight and helps stabilize the body when sitting, acting as one leg of a tripod with the ischial tuberosities.
The Connection Between the Two Bones
The sacrum and the coccyx meet at the sacrococcygeal joint, establishing a direct connection. This junction is classified as a symphysis, where the apex of the sacrum articulates with the base of the coccyx. The joint is reinforced by several strong ligaments, including the anterior and posterior sacrococcygeal ligaments, which provide stability while permitting limited motion.
The movement permitted is slight, primarily consisting of passive flexion and extension. This minor mobility is significant, particularly during childbirth, as the coccyx can move backward to slightly increase the dimensions of the pelvic outlet. The existence of this joint, which may sometimes fuse completely later in life, confirms the coccyx is an independent structure that articulates with the sacrum.