The Thoracic Cage

The thoracic cage (fig. 8.27) consists of the thoracic vertebrae, sternum, and ribs. It forms a more or less conical enclosure for the lungs and heart and provides attachment for the pectoral girdle and upper limb. It has a broad base and a somewhat narrower superior apex; it is rhythmically expanded by the respiratory muscles to create a vacuum that draws air into the lungs. The inferior border of the thoracic cage is formed by a downward arc of the ribs called the costal margin. The ribs protect not only the thoracic organs but also the spleen, most of the liver, and to some extent the kidneys.

Msacr = sacred

41 alae = wings

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 269

Chapter 8 The Skeletal System 269

Thoracic Cage And Pectoral Girdle View

Figure 8.27 The Thoracic Cage and Pectoral Girdle, Anterior View.

Figure 8.27 The Thoracic Cage and Pectoral Girdle, Anterior View.

Sternum

The sternum (breastbone) is a bony plate anterior to the heart. It is subdivided into three regions: the manubrium, body, and xiphoid process. The manubrium43 (ma-NOO-bree-um) is the broad superior portion. It has a superome-dial suprasternal notch (jugular notch), which you can easily palpate between your clavicles (collarbones), and right and left clavicular notches, where it articulates with the clavicles. The body, or gladiolus,44 is the longest part of the sternum. It joins the manubrium at the sternal angle, which can be palpated as a transverse ridge at the point where the sternum projects farthest forward. In some people, however, it is rounded or concave. The second rib attaches here, making the sternal angle a useful landmark for counting ribs in a physical examination. The manubrium and body have scalloped lateral margins where cartilages of the ribs are attached. At the inferior end of the sternum is a small, pointed xiphoid45 (ZIF-oyd) process that provides attachment for some of the abdominal muscles.

43manubrium = handle

44gladiolus = sword

4sxipho = sword + oid = resembling

Ribs

There are 12 pairs of ribs, with no difference between the sexes. Each is attached at its posterior (proximal) end to the vertebral column. A strip of hyaline cartilage called the costal cartilage extends from the anterior (distal) ends of ribs 1 to 7 to the sternum. Ribs 1 to 7 are thus called true ribs. Ribs 8 to 10 attach to the costal cartilage of rib 7, and ribs 11 and 12 do not attach to anything at the distal end but are embedded in thoracic muscle. Ribs 8 to 12 are therefore called false ribs, and ribs 11 and 12 are also called floating ribs for lack of any connection to the sternum.

Ribs 1 to 10 each have a proximal head and tubercle, connected by a narrow neck; ribs 11 and 12 have a head only (fig. 8.28). Ribs 2 to 9 have beveled heads that come to a point between a superior articular facet above and an inferior articular facet below. Rib 1, unlike the others, is a flat horizontal plate. Ribs 2 to 10 have a sharp turn called the angle, distal to the tubercle, and the remainder consists of a flat blade called the shaft. Along the inferior margin of the shaft is a costal groove that marks the path of the intercostal blood vessels and nerve.

Variations in rib anatomy relate to the way different ribs articulate with the vertebrae. Once you observe these articulations on an intact skeleton, you will be better able to understand the anatomy of isolated ribs and vertebrae.

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

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

270 Part Two Support and Movement

f-Tubercle

Head - Neck

Tubercle

Angle f-Tubercle

Head Neck

Superior Inferior Superior Inferior

Costal groove

Tubercle

Head Neck

Angle

Costal groove

Tubercle Rib

Superior Inferior Superior Inferior

Articular facets for vertebral bodies

Shaft

Articular facets for vertebral bodies

Shaft

Figure 8.28 Anatomy of the Ribs. (a) Rib 1 is an atypical flat plate. (b) Typical features of ribs 2 to 10. (c) Appearance of the floating ribs, 11 and 12.

Figure 8.28 Anatomy of the Ribs. (a) Rib 1 is an atypical flat plate. (b) Typical features of ribs 2 to 10. (c) Appearance of the floating ribs, 11 and 12.

Vertebra T1 has a complete superior costal facet on the body that articulates with rib 1, as well as a small inferior costal facet that provides half of the articulation with rib 2. Ribs 2 through 9 all articulate between two vertebrae, so these vertebrae have both superior and inferior costal facets on the respective margins of the body. The inferior costal facet of each vertebra articulates with the superior articular facet of the rib, and the superior costal facet of the next vertebra articulates with the inferior articular facet of the same rib (fig. 8.29a). Ribs 10 through 12 each articulate with a single costal facet on the bodies of the respective vertebrae.

Ribs 1 to 10 each have a second point of attachment to the vertebrae: the tubercle of the rib articulates with the costal facet of the same-numbered vertebra (fig. 8.29b). Ribs 11 and 12 articulate only with the vertebral bodies; they do not have tubercles and vertebrae T11 and T12 do not have costal facets.

Table 8.5 summarizes these variations. Table 8.6 provides a checklist that you can use to review your knowledge of the vertebral column and thoracic cage.

Rib Articulation

Figure 8.29 Articulation of Rib 6 with Vertebrae T5 and T6.

(a) Anterior view. Note the relationships of the articular facets of the rib with the costal facets of the two vertebrae. (b) Superior view. Note that the rib articulates with a vertebra at two points: the costal facet on the vertebral body and the transverse costal facet on the transverse process.

Figure 8.29 Articulation of Rib 6 with Vertebrae T5 and T6.

(a) Anterior view. Note the relationships of the articular facets of the rib with the costal facets of the two vertebrae. (b) Superior view. Note that the rib articulates with a vertebra at two points: the costal facet on the vertebral body and the transverse costal facet on the transverse process.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

  1. Make a table with three columns headed "cervical," "thoracic," and "lumbar." In each column, list the identifying characteristics of each type of vertebra.
  2. Describe how rib 5 articulates with the spine. How do ribs 1 and 12 differ from this and from each other in their modes of articulation?
  3. Distinguish between true, false, and floating ribs. State which ribs fall into each category.
  4. Name the three divisions of the sternum and list the sternal features that can be palpated on a living person.
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Responses

  • Hal
    Do all ribs articulate with two different vertebrae?
    4 years ago
  • may maggot
    How does rib 1 and 12 differs?
    2 months ago

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