Regional Characteristics of Vertebrae

We are now prepared to consider how vertebrae differ from one region of the vertebral column to another and from the generalized anatomy just described. Knowing these variations will enable you to identify the region of the spine from which an isolated vertebra was taken. More importantly, these modifications in form reflect functional differences among the vertebrae.

Cervical Vertebrae

The cervical vertebrae (C1-C7) are the smallest and lightest ones other than the coccygeals. The first two (C1 and C2) have unique structures that allow for head movements (fig. 8.24). Vertebra C1 is called the atlas because it supports the head in a manner reminiscent of the Titan of Greek mythology who was condemned by Zeus to carry the world on his shoulders. It scarcely resembles the typical vertebra; it is little more than a delicate ring surrounding a large vertebral foramen. On each side is a lateral mass with a deeply concave superior articular facet that articulates with the occipital condyle of the skull. A nodding motion of the skull, as in gesturing "yes," causes the occipital condyles to rock back and forth on these facets. The inferior articular facets, which are comparatively flat or only slightly concave, articulate with C2. The lateral masses are connected by an anterior arch and a posterior arch, which bear slight protuberances called the anterior and posterior tubercle, respectively.

Vertebra C2, the axis, allows rotation of the head as in gesturing "no." Its most distinctive feature is a prominent knob called the dens (denz), or odontoid36 process, on its anterosuperior side. No other vertebra has a dens. It begins to form as an independent ossification center during the first year of life and fuses with the axis by the age of 3 to 6 years. It projects into the vertebral foramen of the atlas, where it is nestled in a facet and held in place by a transverse ligament (fig. 8.24c). A heavy blow to the top of the head can cause a fatal injury in which the dens is driven through the foramen magnum into the brainstem. The articulation between the atlas and the cranium is called the atlanto-occipital joint; the one between the atlas and axis is called the atlantoaxial joint.

The axis is the first vertebra that exhibits a spinous process. In vertebrae C2 to C6, the process is forked, or bifid,37 at its tip (fig. 8.25a). This fork provides attachment for the nuchal ligament of the back of the neck. All seven cervical vertebrae have a prominent round transverse foramen in each transverse process. These foramina provide passage and protection for the vertebral arteries, which supply blood to the brain. Transverse foramina occur in no other vertebrae and thus provide an easy means of recognizing a cervical vertebra.

6dens = odont = tooth + oid = resembling

7 bifid = cleft into two parts

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Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

266 Part Two Support and Movement

Axis Vertebrae Superior View

Anterior tubercle Anterior arch Superior articular facet

Transverse foramen

Posterior arch

Posterior tubercle

Dens (odontoid process) Superior articular facet

Transverse foramen Transverse process Inferior articular process

Lamina

Spinous process

Axis of rotation

Dens

Articular Process

Body

Pedicle

Axis

Body

Pedicle

Axis

Axis of rotation

Dens

Transverse ligament

Atlas Axis Articulation

Transverse ligament

Figure 8.24 The Atlas and Axis, Cervical Vertebrae C1 and C2. (a) The atlas, superior view. (fa) The axis, posterosuperior view. (c) Articulation of the atlas and axis and rotation of the atlas. This movement turns the head from side to side, as in gesturing "no." Note the transverse ligament holding the dens of the axis in place.

_Think About It_

How would head movements be affected if vertebrae CI and C2 had the same structure as C3? What is the functional advantage of the lack of a spinous process in CI?

Cervical vertebrae C3 to C6 are similar to the typical vertebra described earlier, with the addition of the transverse foramina and bifid spinous processes. Vertebra C7 is a little different—its spinous process is not bifid, but it is especially long and forms a prominent bump on the lower back of the neck. This feature is a convenient landmark for counting vertebrae. Because it is so conspicuous, C7 is sometimes called the vertebra prominens.

Thoracic Vertebrae

There are 12 thoracic vertebrae (T1-T12), corresponding to the 12 pairs of ribs attached to them. They lack the transverse foramina and bifid processes that distinguish the cervicals, but possess the following distinctive features of their own (fig. 8.25b):

  • The body is somewhat heart-shaped, more massive than in the cervical vertebrae but less than in the lumbar vertebrae.
  • The body has small, smooth, slightly concave spots called costal facets (to be described shortly) for attachment of the ribs.
  • Vertebrae T1 to T10 have a shallow, cuplike transverse costal38 facet at the end of each transverse process. These provide a second point of articulation for ribs 1 to 10. There are no transverse costal facets on T11 and T12 because ribs 11 and 12 attach only to the bodies of the vertebrae.

No other vertebrae have ribs articulating with them. Thoracic vertebrae vary among themselves mainly because of variations in the way the ribs articulate. In most cases, a rib inserts between two vertebrae, so each vertebra contributes one-half of the articular surface. A rib articulates with the inferior costal facet (FASS-et) of the upper vertebra and the superior costal facet of the vertebra below that. This terminology may be a little confusing, but note that the superior and inferior facets are named for

The spinous processes are relatively pointed and angle sharply downward.

3scosta = rib + al = pertaining to

Saladin: Anatomy & I 8. The Skeletal System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 8 The Skeletal System 267

Chapter 8 The Skeletal System 267

Regional Characteristics Vertebrae

Figure 8.25 Typical Cervical, Thoracic, and Lumbar Vertebrae. The left-hand figures are superior views, and the right-hand figures are left lateral views.

List all the features that distinguish vertebra L1 from T12.

Figure 8.25 Typical Cervical, Thoracic, and Lumbar Vertebrae. The left-hand figures are superior views, and the right-hand figures are left lateral views.

List all the features that distinguish vertebra L1 from T12.

their position on the vertebral body, not for which part of the rib's articulation they provide. Vertebrae T1 and T10 to T12, however, have complete costal facets on the bodies for ribs 1 and 10 to 12, which articulate on the vertebral body instead of between vertebrae. Vertebrae T11 and T12, as noted, have no transverse costal facets. These variations will be more functionally understandable after you have studied the anatomy of the ribs, so we will return then to the details of these articular surfaces.

Lumbar Vertebrae

There are five lumbar vertebrae (L1-L5). Their most distinctive features are a thick, stout body and a blunt, squarish spinous process (fig. 8.25c). In addition, their articular processes are oriented differently than on other vertebrae. In thoracic vertebrae, the superior processes face forward and the inferior processes face to the rear. In lumbar vertebrae, the superior processes face medially (like the palms of your hands about to clap), and the inferior processes face laterally, toward the superior processes of the next vertebra. This arrangement makes the lumbar region of the spine especially resistant to twisting. These differences are best observed on an articulated skeleton. Vertebra L1 is an exception to this pattern, as it represents a transition between the thoracic and lumbar pattern. Its superior articular surfaces face dorsally to meet the inferior processes of T12, while its inferior articular surfaces face laterally like those of the rest of the lumbar vertebrae.

Sacrum

The sacrum is a bony plate that forms the dorsal wall of the pelvic cavity (fig. 8.26). It is named for the fact that it

Saladin: Anatomy & I 8. The Skeletal System I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

268 Part Two Support and Movement

268 Part Two Support and Movement

Regional Characteristics Vertebrae
Figure 8.26 The Sacrum and Coccyx. (a) Anterior surface, which faces the viscera of the pelvic cavity. (b) Posterior surface. The processes of this surface can be palpated in the sacral region.

was once considered the seat of the soul.39 In children, there are five separate sacral vertebrae (S1-S5). They begin to fuse around age 16 and are fully fused by age 26.

The anterior surface of the sacrum is relatively smooth and concave and has four transverse lines that indicate where the five vertebrae have fused. This surface exhibits four pairs of large anterior sacral (pelvic) foramina, which allow for passage of nerves and arteries to the pelvic organs. The dorsal surface of the sacrum is very rough. The spinous processes of the vertebrae fuse into a dorsal ridge called the median sacral crest. The transverse processes fuse into a less prominent lateral sacral crest on each side of the median crest. Again on the dorsal side of the sacrum, there are four pairs of openings for spinal nerves, the posterior sacral foramina. The nerves that emerge here supply the gluteal region and lower limb.

A sacral canal runs through the sacrum and ends in an inferior opening called the sacral hiatus (hy-AY-tus). This canal contains spinal nerve roots in life. On each side of the sacrum is an ear-shaped region called the auricular40 (aw-RIC-you-lur) surface. This articulates with a similarly shaped surface on the os coxae and forms the strong, nearly immovable sacroiliac (SAY-cro-ILL-ee-ac) joint. At the superior end of the sacrum, lateral to the median crest, are a pair of superior articular processes that articulate with vertebra L5. Lateral to these are a pair of large, rough, winglike extensions called the alae41 (AIL-ee).

Coccyx

The coccyx42 (fig. 8.26) usually consists of four (sometimes five) small vertebrae, Co1 to Co4, which fuse by the age of 20 to 30 into a single triangular bone. Vertebra Co1 has a pair of hornlike projections, the cornua, which serve as attachment points for ligaments that bind the coccyx to the sacrum. The coccyx can be fractured by a difficult childbirth or a hard fall to the buttocks. Although it is the vestige of a tail, it is not entirely useless; it provides attachment for muscles of the pelvic floor.

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Responses

  • blake
    What are the distinguishing features of cervical, thoracic and lumbar vertebrae?
    5 years ago
  • juliane
    What are the regional characteristics of vertebrae?
    4 years ago
  • Cara Gamgee
    What are the distinguishing characteristics of the twelve thoracic vertebrae?
    4 years ago
  • nicol
    Which of the following characteristics or features belongs to thoracic vertebrae?
    4 years ago
  • ASMARA ELIAS
    How do superior and inferior aticular process of atlas differ than other vertebrae?
    4 years ago
  • goldilocks
    How vertebrae stack to make a spine?
    1 year ago
  • madison
    What is the difference between cervical and thoracic vertebra anatomy?
    11 months ago
  • MAKDA
    How to identify vertebrae?
    5 months ago
  • flavus
    How can we identify its a lumbar vertebrae?
    4 months ago
  • idris
    What are the structural differences between thoracic and lumbar vertebrae?
    4 months ago
  • roberto
    What is the functional advantage of the lack of a spinous in C1?
    2 months ago

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