In adult humans, there are 24 movable vertebrae: 7 cervical, 12 thoracic, and 5 lumbar. The remaining vertebrae include 5 fused sacral, and between 3 and 5 fused caudal.

In children the spinal column contains more vertebrae than in adults; some vertebrae fuse together as the child becomes an adult.

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The spinal column

The spinal column (aka backbone) consists of cervical, thoracic, lumbar, sacral and caudal vertebrae, their intervertebral discs, and the spinal cord with its associated meninges and spinal fluid.

Each vertebra consists of a body, two pedicles, two laminae, one spinous process and two transverse processes.

The body is a more or less cylindrical, massive bone, which rests between the intervertebral discs. These discs are fibrocartilaginous sacs (annulus fibrosus) surrounding a gelatinous center (nucleus pulposus) - think of a jelly donut, and you won't be too far off. These discs act like shock absorbers as well as allowing flexibility.

The pedicles arise from the back of the vertebral body. They form the sides of the vertebral foramen, the opening through which the spinal cord passes.

The laminae curve together from the ends of the pedicles to join into the spinous process (those bumps you can feel on your back are the spinous processes of your vertebrae) and close the arch of the vertebral foramen.

The transverse processes project off to the sides from the pedicles. These are the points on the vertebra where muscles or ribs attach.

The spinal cord is composed of an inner core of gray matter surrounded by white matter. It is divided into 31 segments, each associated with a pair of dorsal and ventral root ganglia, which leave the spinal cord through the opening between adjacent vertebral arches. These roots transport nerve impulses to and from the peripheral nervous system.

The spinal cord is surrounded by the spinal meninges, which are contiguous with the meninges of the brain at the foramen magnum of the skull.

The outermost meningeal layer is the dura mater, a tough fibrous covering which protects the tender spinal cord from contact with the bony surface of the vertebral foramina. The dura mater is surrounded by the epidural space, which contains blood vessels and connective tissue.

The layer under the dura mater is the arachnoid. The space between the dura mater and the arachnoid contains a small amount of fluid to prevent friction.

The innermost meningeal layer is the pia mater, which adheres tightly to the spinal cord. The space between the arachnoid and the pia mater is the subarachnoid space. The subarachnoid space is filled with a webwork of collagen and elastin fibers arising from both meninges. The blood vessels supplying the spinal cord also run through the subarachnoid space, and this is where the cerebrospinal fluid (CSF) circulates.

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