The Sacrum or Sacral Spine

The 33 bones that make up the spine differ considerably in size and structure according to location. There are seven cervical (neck) vertebrae, 12 thoracic (high back), five lumbar (low back), five sacral (near the base of the spine), and four coccygeal (at the base). The five sacral vertebrae are fused to form the sacrum, and the four coccygeal vertebrae are fused to form the coccyx.

S1-S5 - Sacral (saŽkrul) vertebrae:

There are 5 sacral vertebral bones. They are represented by the symbols S1 through S5. The sacral vertebrae are situated in the spinal column just below the lumbar vertebrae and right above the coccyx, which is the lowest segment of the vertebral column. The sacral vertebrae are normally found fused together to form the sacrum.

The sacrum articulates (connects) with the pelvic girdle. In humans it is usually composed of five vertebrae, which fuse in early adulthood. The top of the first (uppermost) sacral vertebra articulates with the last (lowest) lumbar vertebra. The transverse processes of the first three sacral vertebrae are fused to form wide lateral wings, or alae, and articulate with the centre-back portions of the blades of the ilia to complete the pelvic girdle. The sacrum is held in place in this joint, which is called the sacroiliac, by a complex mesh of ligaments. Between the fused transverse processes of the lower sacral vertebrae, on each side, are a series of four openings (sacral foramina); the sacral nerves and blood vessels pass through these openings. A sacral canal running down through the centre of the sacrum represents the end of the vertebral canal; the functional spinal cord ends at about the level of the first sacral vertebra, but its continuation, the filum terminale, can be traced through the sacrum to the first coccygeal vertebra.

The pelvic girdle serves several important functions in the body. It supports the weight of the body from the vertebral column. It also protects and supports the lower organs, including the urinary bladder, the reproductive organs, and the developing fetus in a pregnant woman.

The pelvic girdle differs between men and woman. In a man, the pelvis is more massive and the iliac crests are closer together. In a woman, the pelvis is more delicate and the iliac crests are farther apart. These differences reflect the woman's role in pregnancy and delivery of children. When a child is born, it must pass through its mother's pelvis. If the opening is too small, a cesarean section may be necessary.

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