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.
Anatomy of knee joint.
_1._ Lower end of thigh-bone.
_3._ Knee-pan. _2, 4_ Ligaments
of the knee-pan. _5_. Upper
end of the tibia, or shin-bone.
_6, 12_. Cartilages.]

When protection is required for the organs of the body, or a broad flat
surface for the attachment of the muscles, the bones are expanded into
plates, as in the cranium and shoulder-blades.

The _irregular_ or _mixed_ bones are those which, from their peculiar
shape, cannot be classed among any of the foregoing divisions. Their
structure is similar to the others, consisting of cancellar tissue,
surrounded by a crust of compact matter.

The vertebrae, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two
maxillary, palate, inferior turbinated, and hyoid are known as irregular
bones.

The formation of the joints requires not only bones, but also
cartilages, ligaments, and the synovial membrane, to complete the
articulation. _Cartilage_ is a smooth, elastic substance, softer than
bone, and invested with a thin membrane, called _perichondrium_. When
cartilage is placed upon convex surfaces, the reverse is true. The
_Ligaments_ are white, inelastic, tendinous substances, softer