Fundamentals of Anatomy and Physiology

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central canals and are surrounded by concentric rings of
bone, each layer of which is called a lamella (lah-MELL-
ah). Between two lamellae or rings of bone are several tiny
cavities called lacunae (lah-KOO-nee). Each lacuna
contains an osteocyte or bone cell sus-pended in tissue
fluid. The lacunae are all connected to each other and
ultimately to the larger haversian or central canals by much
smaller canals called canaliculi (kan-ah-LIK-you-lye).
Canals running horizontally to the haversian (central)
canals, also containing blood vessels, are called
Volkmann’s or perforating canals. It is tissue fluid
that circulates through all these canals and bathes the
osteocyte, bringing in oxygen and food and carrying away
waste products and carbon dioxide, keeping the osteocytes
alive and healthy.

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Chapter 7

Cancellous Bone

Cancellous or spongy bone is located at the ends of long

bones and forms the center of all other bones. It consists of

a meshwork of interconnecting sections of bone called

trabeculae (trah-BEK-you-lee), creating the sponge-like

appearance of cancellous bone -(Figure 7-2C). The

trabeculae give strength to the bone without the added

weight of being solid. Each trabecula consists of several

lamellae with osteocytes between the lamellae just as in

compact bone. The spaces between the trabeculae are filled

with bone marrow. Nutrients exit blood vessels in the

marrow and pass by diffusion through the canaliculi of the

lamellae to the osteocytes in the lacunae.

Bone Marrow

The many spaces within certain cancellous bone are filled

with red bone marrow. This marrow is richly sup-plied

with blood and consists of blood cells and their precursors.

The function of red bone marrow is hema-topoiesis, or the

formation of red and white blood cells and blood platelets.

Therefore, blood cells in all stages of

Common Disease, Disorder, or Condition

DISORDERS OF THE SKELETAL SYSTEM

Rickets

Rickets is a disease caused by deficiencies in the minerals calcium and phosphorus or

by deficiencies in vitamin D and sunlight. Vitamin D is necessary for calcium and

phosphorus absorption. The condition causes changes in bones known as rickets in

children and osteomalacia (oss-tee-oh-mah-LAY-she-ah) in adults. The bones fail to

os-sify, resulting in soft, weak bones that are easily broken. Rickets occurs in children

who do not receive adequate exposure to sunlight (sunlight is necessary for vitamin D

production in the body) or whose diets are deficient in vitamin D (milk is a food source of

vitamin D).

Fractures

The breaking of bone or associated cartilage is known as a fracture (Figure 7-5).

-Because bone supports other tissues, a fracture is usually accompanied by injury to

surrounding soft tissues like muscle or connective tissue. Bone fractures are classi-fied

as either open or compound if the bone protrudes through the skin, or closed or simple

if this skin is not perforated. Fractures can also be categorized based on the direction of

the fracture line as transverse (at right angles to a long axis), linear (parallel to a long

axis), or oblique (an angle other than a right angle to a long axis). A greenstick fracture

is an incomplete fracture. The bone is bent but broken on the outer part of the bone. A

comminuted fracture is one in which the bone is shattered into numerous pieces.