The spine, or vertebral column, is made up of numerous individual vertebrae, and extends from the back of the skull to the tip of the tail. It is most flexible at the neck, made up of the cervical vertebrae, and somewhat less flexible in the region located between the rib cage and the pelvis, made up of the lumbar vertebrae. The thoracic vertebrae, located between the cervical and lumbar vertebrae, have the ribs attached to them. There is some movement possible in this region of the spine, but much less than in the cervical and lumbar regions. The sacrum, located between the thoracic vertebrae and the tail, is actually several vertebrae fused together. It is rigidly locked in place between the two sides of the pelvis. The tail, when present, is extremely flexible. It is made up of the coccygeal, or caudal vertebrae. Mammals commonly have seven cervical vertebrae, twelve to eighteen thoracic vertebrae, and five to seven lumbar vertebrae.
At its simplest, a typical vertebra can be visualized as a drum-shaped body with three projections above. Projecting upward is the spinous process, and projecting out to the sides are the transverse processes. At the base of these projections is a hole in the bone through which the spinal cord passes. Other bony projections are present which allow a vertebra to articulate with adjoining vertebrae. Fibrous, elastic Intervertebral discs are located between adjacent vertebrae (except for the joint between the first and second cervical vertebrae).
TOP VIEW (AFTER EILENBERGER)
TOP VIEW (AFTER ELLEhBERGER}
The first two cervical vertebrae are different from the other vertebrae, and they function in ways that have a substantial effect on the surface. The first vertebra, the atlas, is the shortest. It articulates with the back of the skull and allows for only flexion and extension of the skull. This permits the head to tip up and down without the use of any other neck vertebrae, which means that the head can be tipped exclusively at the skull/atlas joint with out any change in form taking place in the rest of the neck. The atlas expands into a bony wing on either side, making it almost as wide as the head—its edges can be felt below the surface, and vary in orientation (obliquity) among the different species.
The second cervical vertebra, the axis, is the longest of all the vertebrae. It permits only side-to-side rotation of the atlas (and therefore the skull) in front of it. This allows the head to rotate side-to-side without the use of the remainder of the neck vertebrae and without changing the form of the rest of the neck. This is most dramatically demonstrated in the giraffe. The axis has a very large spinous process which expands into an upright bony plate that is aligned with the long axis of the body. While not actually sitting below the skin, but rather embedded in muscle, it nevertheless affects surface form on the back of the neck by virtue of its size.
The thoracic vertebrae, especially the front ones between the shoulder blades, can have extremely long spinous processes. For most of them, their tips usually lie just below the skin and create the profile of the animal along its back. In the horse, the spinous process of the fourth vertebra, and those behind it, are all subcutaneous; the fifth is usually the longest. The spinous process of the fourth vertebra is where the nuchal ligament of the neck attaches, and is therefore the point at which the outline of the back of the neck meets the outline of the top of the thorax. Where the neck meets the thorax is an important point to take note of, in all animals, when the head and neck are both raised and lowered. In the ox, the neck profile meets the back profile at the first thoracic spinous process; however, when the neck is lowered, the prominent spinous process of the seventh cervical vertebra in front of it becomes visible.
In the horse, the spinous processes in the shoulder region extend beyond the top edge of the shoulder blade (and its cartilage). They are level with the shoulder blade in the dog, and lie below it in the cat. In the ox, deer, and goat, they are generally at the same level; however, in the bison, the extremely developed spinous processes extend well above the top edge of the shoulder blade.
The lumbar vertebrae have well-developed, horizontally projecting transverse processes. They project outward across the body more than the other vertebrae, and their tips can be quite conspicuous in the ox. The spinous processes above can occasionally be seen on the midline. In the horse and ox, there is limited motion in the lumbar vertebrae. In the carnivores, however, because there are more vertebrae and they are longer and narrower, this region is very flexible. In the ox, the lumbar vertebrae, as well as the thoracic vertebrae, are oriented in a more or less straight line, which gives the animal a fairly straight profile along its back. In contrast, the lumbar region of the carnivores, along with the posterior thoracic vertebrae, arches upward.
Except for the upwardly projecting spinous processes of the front tail vertebrae of the ox, which can be seen on the surface, the individual tail vertebrae of the various animals are buried in muscle and not noticed. Tails are absent in apes and humans, but present in monkeys. Tails can be prehensile (grasping) and hold the entire weight of a moving individual.
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