Physiology and Hygiene for Secondary Schools | Page 8

for carrying on their various activities.
The cells, in order to live, must take in and give out materials, and
water is necessary to both processes. It is also an essential part of the

protoplasm. Deprived of water, cells become inactive and usually die.
Aquatic surroundings are provided for the cells of the body through a
liquid known as the lymph, which is distributed throughout the
intercellular material (Fig. 6). This consists of water containing oxygen
and food substances in solution. Besides supplying these to the cells,
the lymph also receives their wastes. Through the lymph the necessary
conditions for cell life are provided in the body.
*The General Work of Cells.*--In handling the materials derived from
the lymph, the cells carry on three well-defined processes, known as
absorption, assimilation, and excretion.
Absorption is the process of taking water, food, and oxygen into the
cells.
Assimilation is a complex process which results in the addition of the
absorbed materials to the protoplasm. Through assimilation the
protoplasm is built up or renewed.
Excretion is the throwing off of such waste materials as have been
formed in the cells. These are passed into the lymph and thence to the
surface of the body.
Absorption, assimilation, excretion, and also reproduction are
performed by all classes of cells. They are, on this account, referred to
as the general work of cells.
*The Special Work of Cells.*--In addition to the general work which
all cells do in common, each class of cells in the body is able to do
some particular kind of work--a work which the others cannot do or
which they can do only to a limited extent. This is spoken of as the
special work of cells. Examples of the special work of cells are found in
the production of motion by muscle cells and in the secretion of liquids
by gland cells. It may be noted that while the general work of cells
benefits them individually, their special work benefits the body as a
whole. Another example of the special work of cells is found in the
[Fig. 7]

Fig. 7--Cartilage cells, surrounded by the intercellular material which
they have deposited.
*Production of the Intercellular Material.*--Though most of the cells of
the body deposit to a slight extent this material, the greater part of it is
produced by a single class of cells found in bone, cartilage, and
connective tissue. Cartilage, bone, and connective tissue differ greatly
from the other tissues in the amount of intercellular material which they
contain, the difference being due to these cells. In the connective tissue
they deposit the fibrous material so important in holding the different
parts of the body together. In the cartilage they produce the gristly
substance which forms by far its larger portion (Fig. 7). In the bones
they deposit a material similar to that in the cartilage, except that with it
is mixed a mineral substance which gives the bones their hardness and
stiffness.(4) The intercellular material, in addition to connecting the
cells, supplies to certain tissues important properties, such as the
elasticity of cartilage and the stiffness of the bones.
*Nature of the Body Organization.*--The division of labor carried on
by the different organs, as shown in the preceding chapter, is in reality
carried on by the cells that form the organs. To see that this is true we
have only to observe the relation of cells to tissues and of tissues to
organs. The cells form the tissues and the tissues form the organs. This
arrangement enables the special work of different kinds of cells to be
combined in the work of the organ as a whole. This is seen in the hand
which, in grasping, uses motion supplied by the muscle cells, a
controlling influence supplied by the nerve cells, a framework supplied
by the bone cells, and so on. The cells supply the basis for the body
organization and, properly speaking, the body is an organization of
cells(5) (Recall the definition of an organization, page 10.) In this
organization there are to be observed:
1. A definite arrangement of the cells to form the tissues. A tissue is a
group of like cells.
2. A definite arrangement of the tissues in the organ. Each organ
contains the tissues needed for its work.

3. In several instances there is a definite arrangement of organs to form
systems.
4. The body as a whole is made up of organs and systems, together with
the structures necessary for their support and protection.
There now remains a further question for consideration. What is the
one supreme end, or purpose, toward which all the activities of the
body organization are directed? This purpose will naturally have some
relation to the maintenance, or preservation, of the cell group which we
call the body.
*The Maintenance of