Dance Anatomy & Kinesiology

4 Dance Anatomy and Kinesiology

• Flat bones are relatively thin and flat, but often
  slightly curved in shape. These bones commonly
  protect important soft underlying structures (such as
  the brain), and their shape also allows for extensive
  attachment of muscles. Examples include the upper
  portion of the pelvis (ilium) seen in figure 1.1 and
  the ribs, sternum, scapulae, and some of the bones
  of the skull shown in figure 1.4.


• Irregular bones do not fall into the preceding
  three classifications and exhibit complex and varied
  shapes. Their shape is adapted to special purposes;
  and they serve a variety of functions including pro-
  tecting the spinal cord, supporting body weight,
  transmitting loads, providing sites for muscle
  attachment, and facilitating movement. Examples
  include the vertebrae and lower portions of the pelvis
  (ischium and pubis) shown in figures 1.1 and 1.4.


• Sesamoid bones (G. sesamoeides, like sesame) are
  bones that form within a tendon. They help protect
  the tendon from excessive wear due to rubbing against
  the underlying bone, and they change the angle of
  the tendon so that the muscle can produce more
  effective force. Examples include the “kneecap,” or
  patella (figure 1.1), which is encased in the tendon of
  the quadriceps femoris, and two small bones within the
  tendon of the flexor hallucis brevis, located under the
  base of the big toe and discussed in chapter 6. Because
  these sesamoid bones are relatively flat, many texts
  include them in the class of “flat bones” just described,
  while other texts put them in a class of their own.

Structure of Bone

Bone does not have uniform composition. For exam-

ple, the relative percentage of mineralization varies

between bones, as well as within a given bone to help

it better serve its functions. In general, bones have an

outer layer of very dense bone called compact bone

and an inner layer of less dense bone called spongy,

trabecular, or cancellous bone. The compact bone

provides strength and stiffness. Cancellous bone (L. a

grating, lattice) contains many open spaces between

thin processes of bone (trabeculae). These trabecu-

lae (L. trabs, a beam) form a type of latticework that

corresponds to the lines of stress occurring within

the bone. This architecture provides bones with

additional strength and shock-absorbing capacity,

while allowing the bones to be much lighter than if

they were composed solely of compact bone.

Structure of a Typical Long Bone

The compact bone, cancellous bone, and other

structures present in a typical long bone are shown in

figure 1.2. Learning these structures is key for under-

standing bone growth and health. The shaftlike

part, called the diaphysis (G. a growing between),

has thick walls of compact bone and a hollow cavity

called the medullary cavity (L. marrow). The layer

of compact bone thins toward the extremities of

the long bone. The enlarged ends of the bones are

called the epiphyses (G. epi, upon + physis, growth).

These broadened epiphyses provide extensive area

for muscle attachment. They also offer larger surface

areas for articulation with adjacent bones, enhanc-

ing joint stability. The surfaces of the epiphyses that

actually come in contact with opposing bones are

covered with a thin layer of specialized connective

tissue called articular cartilage. Articular cartilage

helps lessen forces and allows joints to move more

smoothly (see Synovial Joints on pp. 12 and 13 for

more information). Rather than housing a hollow

cavity, the epiphyses are filled with cancellous bone.

The spaces of both the cancellous bone and the med-

ullary cavity are filled with a soft, fatty substance called

bone marrow. Some of this marrow (red marrow) is

the type that is vital for making red blood cells.

In bone that is still growing, there is a plate of car-

tilage separating each epiphysis from the diaphysis.

FIGURE 1.2 Structure of a typical long bone (longitudinal

section).