f. Blood Supply. The living bone cells are nourished by a system of blood
vessels and capillaries. In the long bones, for example, blood vessels in the shaft
supply the bone marrow. Branches of blood vessels contained in the periosteum supply
the compact and cancellous bone areas.
g. Microscopic Structure. When bone tissue is examined under a microscope,
it is seen in layers either as a series of flat plates (for cancellous bone) or concentric
cylinders (for compact bone). In compact bones, the series of concentric cylinders are
formed in units called haversian systems. Here, living bone cells lie in minute cavities
called lacunae. The lacunae communicate with each other, and indirectly with a central
haversian canal, through a system of microscopic canals called canaliculi that contain
protoplasmic extensions of bone cells. They are nourished by blood vessels from the
periosteum that enter the compact bone through small pits on the surface. Branches of
these blood vessels penetrate the matrix and enter the central haversian canal in each
CLASSIFICATION OF BONES
Bones may be classified according to shape. This classification is of special
interest to the X-ray specialist.
a. Long Bones. These are long, usually cylindrical, shafts with two expanded
extremities. Long bones are radiographed lengthwise on the film. The humerus
(figure 1-12A) of the arm is an example of a long bone.
b. Short Bones. These bones are short, usually cylindrical shafts, with two
expanded extremities. They usually occur in groups, like the metacarpals of the hand
(figure 1-12B). Generally, short bones are radiographed crosswise on the film.
c. Flat Bones. Flat bones consist of two plates of hard, bony substance with a
layer of spongy, red marrow in between. In the adult, the red marrow is the normal site
for the production of granulocytes (granular leukocytes) and erythrocytes. The scapula
(figure 1-12C) is an example of a flat bone.
d. Irregular Bones. These are bones whose size and shape are modified by
their function and position. The vertebrae (figure 1-12D) are examples of irregular
e. Sesamoid Bones. Sesamoid bones are small bones embedded in tendons
that pass over the joints. In addition to lessening friction, they modify pressure and help
to protect ligaments and tendons. The patella (kneecap) of the knee joint (figure1-13) is
an example of a sesamoid bone. Sesamoid bones are also situated within the palm of
the hand and the plantar surface of the foot.
f. Supernumerary Bones. These are "extra" bones of the skeletal system,
such as an extra vertebra or rib, plus most sesamoid bones.