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The sacrum and coccyx are both bones in the human body. They are considered both part of the vertebral column and part of the pelvic girdle. The sacrum is a large plate-like bone at the base of the lumbar vertebrae, and the hip bones attach to it. The coccyx is a small V-shaped bone below the sacrum, and is is also known as the tailbone. The sacrum and coccyx work together to aid the body in support and provide muscle attachments.
Both the sacrum and the coccyx are made up of separate vertebrae at birth and fuse together later in life. The sacrum is composed of five vertebrae that begin to fuse at age 16 and fully fuse together by age 26. The coccyx is made up of four small vertebrae that fuse together during one’s 20s. It is not uncommon for either bone to be formed from one more or one less vertebrae.
The human body’s ability to stand erect and walk on two feet is aided greatly by the sacrum, on top of which the spinal vertebrae rest. The sacrum helps to support the weight of the upper body. It articulates to the hip bones, forming the sacroiliac joint. The sacrum forms the posterior wall of the pelvic griddle. The large, flattened shape of the sacrum and tight, mainly vertical pelvic girdle allow humans to walk upright.
The sacrum contains several foramina, or holes, that allow for the passage of spinal nerves and arteries. There is a large foramen, the sacral canal, which runs vertically down the center of the sacrum that houses the end of the spinal cord. Four foramina on both the posterior and anterior surfaces of the sacrum allow spinal nerves to branch off from the spinal cord and reach targeted areas.
The coccyx is the vestige of the ancestral tail. It does have very limited function. At the top of the coccyx, there are two lateral horns, or cornua, that serve as attachment points for the ligaments that bind it to the sacrum. The coccyx also provides attachments for the muscles of the pelvic floor. The coccyx can be fractured during difficult childbirth or a hard fall on the buttocks.
There is a slight inward curve of both the sacrum and coccyx that allows humans to sit. The degree of this curve and the overall shape and size of the sacrum differ in men and women. The coccyx in women is more movable and less tilted, to allow for passage of an infant during birth. The sacrum is wider and shorter in women than men, leading to a larger pelvic girdle. This is also necessary during gestation and childbirth.
Examining the sacrum and coccyx is very useful in forensics. The size, shape and curvature of these bones along with the rest of the pelvic girdle can help determine the sex of skeletal remains. It also is helpful for a forensic expert to examine the degree of fusion in the sacrum and coccyx to determine a specimen’s approximate age.