I. FORM, STRUCTURE, ETC., OF THE BONES.

(_See page 269_.)

THE SKELETON, or framework of the "House we live in," is composed of about
200 bones. [Footnote: The precise number varies in different periods of
life. Several which are separated in youth become united in old age. Thus
five of the "false vertebre" at the base of the spine early join in one
great bone--the sacrum; while four tiny ones below it often run into a
bony mass--the coccyx (Fig. 6); in the child, the sternum is composed of
eight pieces, while in the adult it consists of only three. While,
however, the number of the bones is uncertain, their relative length is so
exact that the length of the entire skeleton, and thence the height of the
man, can be obtained by measuring a single one of the principal bones.
Fossil bones and those found at Pompeii have the same proportion as our
own.]

USES AND FORMS OF THE BONES.--They have three principal uses: 1. To
protect the delicate organs; [Footnote: An organ is a portion of the body
designed for a particular use, called its _function_. Thus the heart
circulates the blood; the liver produces the bile.] 2. To serve as levers
on which the muscles may act to produce motion; and 3. To preserve the
shape of the body.

Bones differ in form according to the uses they subserve. For convenience
in walking, some are long; for strength and compactness, some are short
and thick; for covering a cavity, some are flat; and for special purposes,
some are irregular. The general form is such as to combine strength and
lightness. For example, all the long bones of the limbs are round and
hollow, thus giving with the same weight a greater strength, [Footnote: