REPAIR OF THE BONES.--When a bone is broken, the blood at once oozes out
of the fractured ends. This soon gives place to a watery fluid, which in a
fortnight thickens to a gristly substance, strong enough to hold them in
place. Bone matter is then slowly deposited, which in five or six weeks
will unite the broken parts. Nature, at first, apparently endeavors to
remedy the weakness of the material by excess in the quantity, and so the
new portion is larger than the old. But the extra matter will be gradually
absorbed, sometimes so perfectly as to leave no trace of the injury. (See
p. 271.)

A broken limb should be held in place by splints, or a plaster cast, to
enable this process to go on uninterruptedly, and also lest a sudden jar
might rupture the partially mended break. For a long time, the new portion
consists largely of animal matter, and so is tender and pliable. The
utmost care is therefore necessary to prevent a malformation.

THE JOINTS are packed with a soft, smooth cartilage, or gristle, which
fits so perfectly as to be airtight. Upon convex surfaces, it is thickest
at the middle, and upon concave surfaces, it is thickest at the edge, or
where the wear is greatest. In addition, the ends of the bones are covered
with a thin membrane, the _synovial_ (_sun_, with; _ovum_,
an egg), which secretes a viscid fluid, not unlike the white of an egg.
This lubricates the joints, and prevents the noise and wear of friction.
The body is the only machine that oils itself.

The bones which form the joint are tied with stout ligaments (_ligo_,
I bind), or bands, of a smooth, silvery white tissue, [Footnote: The
general term _tissue_ is applied to the various textures of which the
organs are composed. For example, the osseous tissue forms the bones; the