nces needed by the cells, and preserve the life of the body
(Fig. 92).

In the chapters which follow, we are to consider the problem of adjusting
the body to and of bringing it into proper relations with its
surroundings.

PART II: MOTION, COORDINATION, AND SENSATION

CHAPTER XIV - THE SKELETON

One necessary means of establishing proper relations between the body and
its surroundings is _motion_.(77) Not only can the body move itself from
place to place, but it is able to move surrounding objects as well. In the
production of motion three important systems are employed--the muscular
system, the nervous system, and a system of mechanical devices which are
found mainly in the skeleton. The muscular system supplies the energy for
operating the mechanical devices, while the nervous system controls the
movements.(78) Although the skeleton serves other purposes, such as giving
shape to the body and protecting certain organs, its main use is that of
an aid in the production of motion.

*Skeleton Tissues.*--The tissues employed in the construction of the
skeleton are the osseous, the cartilaginous, and the connective tissues.
These are known as the supporting tissues of the body. They form the
bones, supply the elastic pads at the ends of the bones, and furnish
strong bands, called ligaments, for fastening the bones together. The
skeleton forms about 16 per cent of the weight of the body. Its tissues,
being of a more durable nature than the rest of the body, do not so
readily decay. Especially is this true of the osseous tissue, which may be
preserved indefinitely, after removal from the body, by simply keeping it
dry.

*The Bones.*--The separate units, or parts, of which the skeleton is
constructed are called bones. They are the hard structures that can be
felt in all parts of the body, and they comprise nearly the entire amount
of material found in the prepared skeleton. As usually estimated, the
bones are 208 in number. They vary greatly in size and shape in different
parts of the body.

*Composition and Properties of Bones.*--The most noticeable and important
properties of the bones are those of hardness, stiffness, and toughness.
Upon these properties the uses of the bones depend. These properties may,
in turn, be shown to depend upon the presence in osseous tissue of two
essentially different kinds of substance, known as the _animal matter_ and
the _mineral matter_. If a bone is soaked in an acid,