art of the structure of the skin.

The number of distinct bones varies at different periods of life. It is
greater in childhood than in adults, for many bones which are then
separate, to allow growth, afterwards become gradually united. In early
adult life, for instance, the skull contains 22 naturally separate bones,
but in infancy the number is much greater, and in old age far less.

The bones of the body thus arranged give firmness, strength, and
protection to the soft tissues and vital organs, and also form levers for
the muscles to act upon.

28. Chemical Composition of Bone. The bones, thus forming the
framework of the body, are hard, tough, and elastic. They are twice as
strong as oak; one cubic inch of compact bone will support a weight of
5000 pounds. Bone is composed of earthy or mineral matter
(chiefly in the form of lime salts), and of animal matter
(principally gelatine), in the proportion of two-thirds of the former to
one-third of the latter.

[Illustration: Fig. 10.--The Skeleton.]

The proportion of earthy to animal matter varies with age. In infancy the
bones are composed almost wholly of animal matter. Hence, an infant's
bones are rarely broken, but its legs may soon become misshapen if walking
is allowed too early. In childhood, the bones still contain a larger
percentage of animal matter than in more advanced life, and are therefore
more liable to bend than to break; while in old age, they contain a
greater percentage of mineral matter, and are brittle and easily broken.

Experiment 3. _To show the mineral matter in bone_. Weigh a large
soup bone; put it on a hot, clear fire until it is at a red heat. At
first it becomes black from the carbon of its organic matter, but at
last it turns white. Let it cool and weigh again. The animal matter has
been burnt out, leaving the mineral or earthy part, a white, brittle
substance of exactly the same shape, but weighing only about two-thirds
as much as the bone originally weighed.

Experiment 4. _To show the animal matter in bone_. Add a
teaspoonful of muriatic acid to a pint of water, and place the mixture
in a shallow earthen dish. Scrape and clean a chicken's leg bone, part
of a sheep's rib, or any other small, thin bone. Soak the bone in the
acid mixture for a few days. The earthy or mineral matter is slowly
dissolved, and the bone, although retaining its original form, loses its
rigidity, and becomes pliable, and so soft