Physiology and Hygiene for Secondary Schools | Page 7

associated, and for this reason protoplasm is called the physical
basis of life. Its organization into separate bits, or cells, is necessary to
the life activities that take place within it.
*Structure of the Cell.*--Though all portions of the cell are formed
from the protoplasm, this essential substance differs both in structure
and in function at different places in the cell. For this reason the cell is
looked upon as a complex body having several distinct parts. At or near
the center is a clear, rounded body, called the nucleus. This plays some
part in the nourishment of the cell and also in the formation of new
cells. If it be absent, as is sometimes the case, the cell is short-lived and

unable to reproduce itself. The variety of protoplasm contained in the
nucleus is called the nucleoplasm.
[Fig. 4]
Fig. 4--Diagram of a typical cell (after Wilson). 1. Main body. 2.
Nucleus. 3. Attraction sphere. 4. Food particles and waste. 5. Cell-wall.
6. Masses of active material found in certain cells, called plastids.
Surrounding the nucleus is the main body of the cell, sometimes
referred to as the "protoplasm." Since the protoplasm forms all parts of
the cell, this substance is more properly called the cytoplasm, or cell
plasm. Surrounding and inclosing the cytoplasm, in many cells, is a
thin outer layer, or membrane, which affords more or less protection to
the contents of the cell. This is usually referred to as the cell-wall. A
fourth part of the cell is also described, being called the attraction
sphere. This is a small body lying near the nucleus and coöperating
with that body in the formation of new cells. Food particles, wastes,
and other substances may also be present in the cytoplasm. The parts of
a typical cell are shown in Fig. 4.
*Importance of the Cells.*--The cells must be regarded as the living,
working parts of the body. They are the active agents in all of the
tissues, enabling them to serve their various purposes. Working through
the tissues, they build up the body and carry on its different activities.
They are recognized on this account as the units of structure and of
function, and are the "individuals" in the body organization. Among the
most important and interesting of the activities of the cells are those by
which they build up the body, or cause it to grow.
*How the Cells enable the Body to Grow.*--Every cell is able to take
new material into itself and to add this to the protoplasm. This tends to
increase the amount of the protoplasm, thereby causing the cells to
increase in size. A general increase in the size of the cells has the effect
of increasing the size of the entire body, and this is one way by which
they cause it to grow. There is, however, a fixed limit, varying with
different cells, to the size which they attain, and this is quite low. (The
largest cells are scarcely visible to the naked eye.) Any marked increase

in the size of the body must, therefore, be brought about by other
means. Such a means is found in the formation of new cells, or cell
reproduction. The new cells are always formed by and from the old
cells, the essential process being known as cell-division.
[Fig. 5]
Fig. 5--Steps in cell-division (after Wilson). Note that the process
begins with the division of the attraction sphere, then involves the
nucleus, and finally separates the main body.
*Cell-Division.*--By dividing, a single cell will, on attaining its growth,
separate into two or more new cells. The process is quite complex and
is imperfectly understood. It is known, however, that the act of
separation is preceded by a series of changes in which the attraction
sphere and the nucleus actively participate, and that, as a result of these
changes, the contents of the old cell are rearranged to form the new
cells. Some of the different stages in the process, as they have been
studied under the microscope, are indicated in Fig. 5.
Gradually, through the formation of new cells and by the growth of
these cells after they have been formed, the body attains its full size.
When growth is complete, cell reproduction is supposed to cease except
where the tissues are injured, as in the breaking of a bone, or where
cells, like those at the surface of the skin, are subject to wear. Then new
material continues to be added to the protoplasm throughout life, but in
amount only sufficient to replace that lost from the protoplasm as
waste.
[Fig. 6]
Fig. 6--A tumbler partly filled with marbles covered with water,
suggesting the relations of the cells to the lymph.
*Cell Surroundings.*--All cells are said to be aquatic. This means
simply that they require water