hand, the wrist, the forearm, the elbow, and the upper arm are the main
divisions of each of the upper extremities. The toes, the foot, the ankle,
the lower leg, the knee, and the thigh are the chief divisions of each of
the lower extremities. The head, which is joined to the trunk by the neck,
has such interesting parts as the eyes, the ears, the nose, the jaws, the
cheeks, and the mouth. The entire body is inclosed in a double covering,
called the _skin_, which protects it in various ways.

*The Tissues.*—After examining the external features of the body, we
naturally inquire about its internal structures. These are not so easily
investigated, and much which is of interest to advanced students must be
omitted from an elementary course. We may, however, as a first step in
this study, determine what kinds of materials enter into the construction
of the body. For this purpose the body of some small animal should be
dissected and studied. (See observation at close of chapter.) The
different materials found by such a dissection correspond closely to the
substances, called _tissues_, which make up the human body. The main
tissues of the body, as ordinarily named, are the _muscular_ tissue, the
_osseous_ tissue, the _connective_ tissue, the _nervous_ tissue, the
_adipose_ tissue, the _cartilaginous_ tissue, and the _epithelial_ and
_glandular_ tissue. Most of these present different varieties, making all
together some fifteen different kinds of tissues that enter into the
construction of the body.(2)

*General Purposes of 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.