ossification at distinct points, called _centers of ossification_, are
all important subjects, requiring the student's careful attention. The
bones are protected by an external membranous envelope, which, from its
situation is called the _periosteum_. The bones are divided into four
classes, _long, short, flat_ and _irregular_, being thus adapted to
subserve a variety of purposes.

The Long Bones are found in the limbs, where they act as levers to
sustain the body and aid in locomotion. Each_long_ bone is composed of a
cylinder, known as the _shaft_, and two _extremities_. The shaft is
hollow, its wails being _thickest_ in THE middle and growing thinner
toward the extremities. The _extremities_ are usually considerably
enlarged, for convenience of connection with other bones, and to afford
a broad surface for the attachment of muscles. The clavical, humerus,
radius, ulna, femur, tibia, fibula, the bones of the metacarpus,
metatarsus and the phalanges, are classed as long bones.

Where the principal object to be attained is strength, and the motion of
the skeleton is limited, the individual bones are short and compressed,
as the bones of the carpus and tarsus. The structure of these bones is
spongy, except at the surface, where there is a thin crust of compact
matter.

[Illustration: Fig. 17.
Anatomy of a joint, _1, 1_.
Bones of a joint. _2, 2_. Cartilage.
_3, 3, 3, 3_. Synovial
membrane.]

[Illustration: Fig. 18.