General Features of Bones

Knowing the terms used to describe a long bone will help you to understand the anatomy of the other types.

Figure 7.2a shows a longitudinal section through a long bone. You will note immediately that much of it is composed of a cylinder of dense white osseous tissue; this is called compact (dense) bone. The cylinder encloses a space called the medullary (MED-you-lerr-ee) cavity, which contains bone marrow. At the ends of the bone, the central space is occupied by a more loosely organized form of osseous tissue called spongy (cancellous) bone. The skeleton is about three-quarters compact bone and one-quarter spongy bone by weight. Spongy bone is found at the ends of the long bones and in the middle of nearly all others. It is always enclosed by more durable compact bone. In flat bones of the skull, two layers of compact bone enclose a middle layer of spongy bone like a sandwich (fig. 7.2b). The spongy layer is called the diploe4 (DIP-lo-ee). A moderate blow to the skull can fracture the outer layer of compact bone, but the diploe may absorb the impact and leave the inner layer of compact bone unharmed.

The principal features of a long bone are its shaft, called the diaphysis5 (dy-AF-ih-sis), and an expanded head at each end called the epiphysis6 (eh-PIF-ih-sis).

4 diplo = double

5 dia = across + physis = growth; originally named for a ridge on the shaft of the tibia

Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill

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

220 Part Two Support and Movement

Epiphysis

Diaphysis

Epiphysis (a)

220 Part Two Support and Movement

Epiphysis

Diaphysis

Compact Bone Marrow Cavity
  • Epiphyseal line
  • Spongy bone
  • Compact bone
  • Medullary cavity Yellow bone marrow

Perforating fibers

Periosteum Nutrient vessel Nutrient foramen

Endosteum

- Articular cartilage

Suture

Outer compact bone -

Suture

Outer compact bone -

Trabeculae

Inner compact bone

Figure 7.2 Anatomy of Bones. (a) A long bone, the tibia. (b) Flat bones of the cranium.

What is the functional significance of a long bone being wider at the epiphyses than at the diaphysis?

The diaphysis provides leverage, while the epiphysis is enlarged to strengthen the joint and provide added surface area for the attachment of tendons and ligaments. The joint surface where one bone meets another is covered with a layer of hyaline cartilage called the articular cartilage. Together with a lubricating fluid secreted between the bones, this cartilage enables a joint to move far more easily than it would if one bone rubbed directly against the other. Blood vessels penetrate into the bone through minute holes called nutrient foramina (for-AM-ih-nuh); we will trace where they go when we consider the histology of bone.

Externally, a bone is covered with a sheath called the periosteum.7 This has a tough, outer fibrous layer of collagen and an inner osteogenic layer of bone-forming cells described later in the chapter. Some collagen fibers of the outer layer are continuous with the tendons that bind muscle to bone, and some penetrate into the bone matrix as perforating (Sharpey8) fibers. The periosteum thus provides strong attachment and continuity from muscle to tendon to bone. The osteogenic layer is important to the growth of bone and healing of fractures. There is no periosteum over the articular cartilage. The internal surface of a bone is lined with endosteum,9 a thin layer of reticular connective tissue and osteogenic cells that give rise to other types of bone cells.

In children and adolescents, an epiphyseal (EP-ih-FIZZ-ee-ul) plate of hyaline cartilage separates the marrow spaces of the epiphysis and diaphysis (fig 7.2a). On X rays, it appears as a transparent line at the end of a long bone (see fig. 7.11). The epiphyseal plate is a zone where the bones elongate by a growth process detailed later in the chapter. In adults, the epiphyseal plate is depleted and the bones no longer grow in length, but an epiphyseal line marks where the plate used to be.

8 William Sharpey (1802-80), Scottish histologist

Saladin: Anatomy & I 7. Bone Tissue I Text I I © The McGraw-Hill

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

Before You Go On

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

  1. Name five tissues found in a bone.
  2. List three or more functions of the skeletal system other than supporting the body and protecting some of the internal organs.
  3. Name the four bone shapes and give an example of each.
  4. Explain the difference between compact and spongy bone, and describe their spatial relationship to each other.
  5. State the anatomical terms for the shaft, head, growth zone, and fibrous covering of a long bone.

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