the Tissues.*--The tissues, first of all, _form the
body_. As a house is constructed of wood, stone, plaster, iron, and other
building materials, so is the body made up of its various tissues. For
this reason the tissues have been called the _building materials_ of the
body.

In addition to forming the body, the tissues supply the means through
which its work is carried on. They are thus the _working materials_ of the
body. In serving this purpose the tissues play an active role. All of them
must perform the activities of growth and repair, and certain ones (the
so-called active tissues) must do work which benefits the body as a whole.

*Purposes of the Different Tissues.*--In the construction of the body and
also in the work which it carries on, the different tissues are made to
serve different purposes. The osseous tissue is the chief substance in the
bony framework, or skeleton, while the muscular tissue produces the
different movements of the body. The connective tissue, which is
everywhere abundant, serves the general purpose of connecting the
different parts together. Cartilaginous tissue forms smooth coverings over
the ends of the bones and, in addition to this, supplies the necessary
stiffness in organs like the larynx and the ear. The nervous tissue
controls the body and brings it into proper relations with its
surroundings, while the epithelial tissue (found upon the body surfaces
and in the glands) supplies it with protective coverings and secretes
liquids. The adipose tissue (fat) prevents the too rapid escape of heat
from the body, supplies it with nourishment in time of need, and forms
soft pads for delicate organs like the eyeball.

*Properties of the Tissues.*--If we inquire how the tissues are able to
serve such widely different purposes, we find this answer. The tissues
differ from one another both in composition and in structure and, on this
account, differ in their properties.(3) Their different properties enable
them to serve different purposes in the body. Somewhat as glass is adapted
by its transparency, hardness, and toughness to the use made of it in
windows, the special properties of the tissues adapt them to the kinds of
service which they perform. Properties that adapt tissues to their work in
the body are called _essential_ properties. The most important of these
essential properties are as follows:

1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular
tissue, contract