Coccygeus muscle

Origin and insertion

Coccygeus is a triangular musculotendinous sheet of muscle containing an apex and a base. The apex originates from the pelvic surface of the ischial spine, including the tip. Most of the muscle fibers insert via its base into the lateral margins of the coccyx and the inferior end of sacrum, at the level of the fifth segment (S5).

The remaining fibers converge in the midline, completing the triangular shape. The aponeurotic portion of the muscle forms the sacrospinous ligament of the pelvis. Therefore, this ligament is considered to be a remnant or degenerated part of the coccygeus muscle.

However, the coccygeus might not be muscular at all in certain individuals. In these cases, coccygeus is completely tendinous and fused with the sacrospinous ligament of the pelvis.

Relations

Coccygeus lies within the plane of the pelvic outlet. It is located posterosuperiorly to iliococcygeus, which is a part of the levator ani muscle complex. Some resources argue that coccygeus is actually a part of levator ani, rather than a distinct muscle.

Coccygeus is also in contact with the lower border of piriformis, forming a canal in the greater sciatic foramen through which pelvic arteries and nerves travel between the pelvis and gluteal region. The most important neurovascular structures passing through this canal are the internal pudendal artery (pelvic portion), inferior gluteal artery and pudendal nerve.

In addition, coccygeus is also directly pierced by several neurovascular structures. These include the nerves to levator ani, which then course across the anterior surface of coccygeus, and the perineal branch of the spinal nerve L4. The latter eventually provides cutaneous innervation to the skin between the anus and coccyx.

The anterior (ventral) ramus of spinal nerve S5 which contributes to the coccygeal plexus also pierces coccygeus from below. Together with a branch of the anterior ramus of spinal nerve S4 and anterior ramus of the coccygeal nerve (spinal nerve Co), the anterior ramus of spinal nerve S5 forms a trunk on the pelvic surface of the coccygeus muscle. This trunk continues to descend along the pelvic surface, curves around the lateral margin of coccygeus and pierces it on its way towards the pelvis.

Innervation

Coccygeus is primarily innervated by the anterior ramus of spinal nerve S4 (a.k.a. fourth sacral nerve), but receives minor contributions from spinal nerve S5 also. 

Blood supply

Coccygeus receives arterial blood via three branches of the internal iliac artery; inferior vesical, inferior gluteal and internal pudendal arteries.

Functions

The main function of coccygeus is mainly supportive. It takes part in forming a relatively horizontal pelvic diaphragm, assisting levator ani to support the pelvic viscera. The contraction of coccygeus pulls the anatomical structures within the pelvis and perineum anterosuperiorly, helping levator ani to occlude the physiological hiatuses and prevent the prolapse of internal organs.

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Coccygeus also weakly aids puborectalis to control defecation and urination, especially when intra-abdominal pressure is raised, such as while coughing. During childbirth, and sometimes defecation, the coccyx bone is temporarily pushed posteriorly. After the given process is complete, contraction of coccygeus flexes the coccyx and pulls it anteriorly to the initial position.