Which vertebrate lacks a bony spine

The arch consists of bilateral pedicles, cylindrical processes of bone that connect the arch to the body, and bilateral lamina, flat, bone segments form most of the arch, connecting the transverse and spinous processes. A typical vertebra also contains four articular processes, two superior and two inferior, which contact the inferior and superior articular processes of adjacent vertebrae, respectively. The point at which superior and articular facets meet is known as a facet, or zygapophyseal, joint.

These maintain vertebral alignment, control the range of motion, and are weight-bearing in certain positions. The spinous process projects posteriorly, and often inferiorly from the vertebral arch and may overlap the inferior vertebrae to various degrees, depending on the region of the spine. Lastly, the two transverse processes project laterally from the vertebral arch in a symmetrical fashion.

Typical cervical vertebrae have several features distinct from those typical of thoracic or lumbar vertebrae. The most notable distinction is the presence of one foramen, in each transverse process.

These transverse foramina encircle the vertebral arteries and veins. This is true of all cervical vertebrae except C7, whose transverse foramina contain only accessory veins. The spinous process of cervical vertebrae increases as the spinal column descends. Inferiorly, C1 articulates with the C2 vertebra, and so on.

Below these are the 12 thoracic vertebrae, designated T1—T The lower back contains the L1—L5 lumbar vertebrae. The single sacrum, which is also part of the pelvis, is formed by the fusion of five sacral vertebrae. Similarly, the coccyx, or tailbone, results from the fusion of four small coccygeal vertebrae. However, the sacral and coccygeal fusions do not start until age 20 and are not completed until middle age. An interesting anatomical fact is that almost all mammals have seven cervical vertebrae, regardless of body size.

This means that there are large variations in the size of cervical vertebrae, ranging from the very small cervical vertebrae of a shrew to the greatly elongated vertebrae in the neck of a giraffe. In a full-grown giraffe, each cervical vertebra is 11 inches tall. When the load on the spine is increased, by carrying a heavy backpack for example, the curvatures increase in depth become more curved to accommodate the extra weight.

They then spring back when the weight is removed. Disorders associated with the curvature of the spine include kyphosis an excessive posterior curvature of the thoracic region , lordosis an excessive anterior curvature of the lumbar region , and scoliosis an abnormal, lateral curvature, accompanied by twisting of the vertebral column.

Developmental anomalies, pathological changes, or obesity can enhance the normal vertebral column curves, resulting in the development of abnormal or excessive curvatures Figure 6. Kyphosis, also referred to as humpback or hunchback, is an excessive posterior curvature of the thoracic region.

This can develop when osteoporosis causes weakening and erosion of the anterior portions of the upper thoracic vertebrae, resulting in their gradual collapse Figure 6. Lordosis, or swayback, is an excessive anterior curvature of the lumbar region and is most commonly associated with obesity or late pregnancy.

The accumulation of body weight in the abdominal region results an anterior shift in the line of gravity that carries the weight of the body.

This causes in an anterior tilt of the pelvis and a pronounced enhancement of the lumbar curve. Scoliosis is an abnormal, lateral curvature, accompanied by twisting of the vertebral column. Compensatory curves may also develop in other areas of the vertebral column to help maintain the head positioned over the feet. Scoliosis is the most common vertebral abnormality among girls.

The cause is usually unknown, but it may result from weakness of the back muscles, defects such as differential growth rates in the right and left sides of the vertebral column, or differences in the length of the lower limbs.

When present, scoliosis tends to get worse during adolescent growth spurts. Although most individuals do not require treatment, a back brace may be recommended for growing children.

In extreme cases, surgery may be required. Osteoporosis is a common age-related bone disease in which bone density and strength is decreased. Watch this video to get a better understanding of how thoracic vertebrae may become weakened and may fracture due to this disease.

How may vertebral osteoporosis contribute to kyphosis? Within the different regions of the vertebral column, vertebrae vary in size and shape, but they all follow a similar structural pattern. A typical vertebra will consist of a body, a vertebral arch, and seven processes Figure 6. The body is the anterior portion of each vertebra and is the part that supports the body weight. Because of this, the vertebral bodies progressively increase in size and thickness going down the vertebral column.

The bodies of adjacent vertebrae are separated and strongly united by an intervertebral disc. The vertebral arch forms the posterior portion of each vertebra.

It consists of four parts, the right and left pedicles and the right and left laminae. Each pedicle forms one of the lateral sides of the vertebral arch. The pedicles are anchored to the posterior side of the vertebral body. Each lamina forms part of the posterior roof of the vertebral arch.

On the left and right lateral sides of each vertebra are the short, triangular transverse processes. The transverse processes form important connection points for many muscles, including the rotatores and multifidus muscles that extend and rotate the trunk. Unlike the cervical vertebrae in the neck, the lumbar vertebrae lack the transverse foramina in the transverse processes, and also lack facets to either side of the body.

The fifth lumbar vertebra is distinct from the L vertebrae in being much larger on its front side than in the back. Its spinous process, on the other hand, is smaller than in the other lumbar vertebrae with a wide, four-sided shape that comes to a rough edge and a thick notch.

By: Tim Taylor. Last Updated: Jul 16,