There are normally thirty-three (33) vertebrae in humans, including the five that are fused to form the sacrum (the others are separated by intervertebral discs) and the four coccygeal bones which form the tailbone. The upper three regions comprise the remaining 24, and are grouped under the names cervical (7 vertebrae), thoracic (12 vertebrae) and lumbar (5 vertebrae), according to the regions they occupy. This number is sometimes increased by an additional vertebra in one region, or it may be diminished in one region, the deficiency often being supplied by an additional vertebra in another. The number of cervical vertebrae is, however, very rarely increased or diminished.
With the exception of the first and second cervical, the true or movable vertebrae (the upper three regions) present certain common characteristics which are best studied by examining one from the middle of the thoracic region.
A typical vertebra consists of two essential parts: an anterior (front) segment, which is the vertebral body; and a posterior part – the vertebral (neural) arch – which encloses the vertebral foramen. The vertebral arch is formed by a pair of pedicles and a pair of laminae, and supports seven processes, four articular, two transverse, and one spinous, the latter also being known as the neural spine.
When the vertebrae are articulated with each other, the bodies form a strong pillar for the support of the head and trunk, and the vertebral foramina constitute a canal for the protection of the medulla spinalis (spinal cord). In between every pair of vertebrae are two apertures, the intervertebral foramina, one on either side, for the transmission of the spinal nerves and vessels.
Two transverse processes and one spinous process are posterior to (behind) the vertebral body. The spinous process comes out the back, one transverse process comes out the left, and one on the right. The spinous processes of the cervical and lumbar regions can be felt through the skin. Superior and inferior articular facets on each vertebra act to restrict the range of movement possible. These facets are joined by a thin portion of the neural arch called the pars interarticularis.
These are generally small and delicate. Their spinous processes are short (with the exception of C2 and C7, which have palpable spinous processes), and often split. Numbered top-to-bottom from C1-C7, atlas (C1) and axis (C2), are the vertebrae that allow the neck and head so much movement. For the most part, the atlanto-occipital joint allows the skull to move up and down, while the atlanto-axial joint allows the upper neck to twist left and right. The axis also sits upon the first intervertebral disk of the spinal column. All mammals except manatees and sloths have seven cervical vertebrae, whatever the length of the neck.
Their transverse processes have surfaces that articulate with the ribs. Some rotation can occur between the thoracic vertebrae, but their connection with the rib cage prevents much flexion or other excursion. They may also be known as 'dorsal vertebrae', in the human context.
These vertebrae are very robust in construction, as they must support more weight than other vertebrae. They allow significant flexion and extension, moderate lateral flexion (sidebending), and a small degree of rotation. The discs between these vertebrae create a lumbar lordosis (curvature that is concave posteriorly) in the human spine.
There are 5 vertebrae (S1-S5). They are fused in maturity, with no intervertebral discs.
There are 3-5 vertebrae (Co1-Co5), with no intervertebral discs. Many animals have a greater number of "tail vertebrae" and, in animals, they are more commonly known as "caudal vertebrae." Pain at the coccyx (tailbone) is known as coccydynia.
The less dense tissue that separates the sclerotome segments develop into the intervertebral discs.
The notochord disappears in the sclerotome (vertebral body) segments, but persists in the region of the intervertebral discs as the nucleus pulposus. The nucleus pulposus and the fibers of the annulus fibrosus make up the intervertebral disc.
The primary curves (thoracic and sacral curvatures) form during fetal development. The secondary curves develop after birth. The cervical curvature forms as a result of lifting the head and the lumbar curvature forms as a result of walking.
There are various defects associated with vertebral development. Scoliosis can result from improper fusion of the vertebrae. In Klippel-Feil anomaly patients have two or more cervical vertebrae that are fused together, along with other associated birth defects. One of the most serious defects is failure of the vertebral arches to fuse. This results in a condition called spina bifida. There are several variations of spina bifida that reflect the severity of the defect.