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 vertebræ, 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 than
cartilage, but harder than membrane. Their function is to bind together
the bones. The _Synovial Membrane_ covers the cartilages, and is then
reflected upon the ligaments, thus forming a thin, closed sac, called
the _synovial capsule._