What is the nature of the body organization? What are the individual
parts, or units, that make it up? What general work do these carry on and
upon what basis do they practice the division of labor? The answers to
these questions will suggest the main problems in the study of the body.
[Fig. 3]
Fig. 3--Diagram showing the relation of the cells and the intercellular
material. _C._ Cells. _I._ Intercellular material.
*Complex Nature of the Tissues.*--To the unaided eye the tissues have the
appearance of simple structures. The microscope, however, shows just the
reverse to be true. When any one of the tissues is suitably prepared and
carefully examined with this instrument, at least two classes of materials
can be made out. One of these consists of minute particles, called
_cells_; the other is a substance lying between the cells, known as the
_intercellular material_ (Fig. 3). The cells and the intercellular
material, though varying in their relative proportions, are present in all
the tissues.
*The Body a Cell Group.*--The biologist has found that the bodies of all
living things, plants as well as animals, consist either of single cells
or of groups of cells. The single cells live independently of one another,
but the cells that form groups are attached to, and are more or less
dependent upon, one another. In the first condition are found the very
lowest forms of life. In the second, life reaches its greatest
development. The body of man, which represents the highest type of life,
is recognized as a group of cells. In this group each cell is usually
separate and distinct from the others, but is attached to them, and is
held in place by the intercellular material.
*Protoplasm, the Cell Substance.*--The cell is properly regarded as an
_organized_ bit of a peculiar material, called _protoplasm_. This is a
semi-liquid and somewhat granular substance which resembles in appearance
the white of a raw egg. Its true nature and composition are unknown,
because any attempt to analyze it kills it, and dead protoplasm is
essentially different from living protoplasm. It is known, however, to be
a highly complex substance and to undergo chemical change readily. It
appears to be the only kind of matter with which life is ever 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 cooeperating 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 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 Life.*--The preservation of any cell group in its
natural condition, whether it be plant or animal, is accomplished through
keeping it alive. If life ceases, the group quickly disintegrates and its
elements become scattered, a fact which is verified through everyday
observation. Though the nature of life is unknown, it may be looked upon
as the organizer and preserver of the protoplasm. But in preserving the
protoplasm it also preserves the entire cell group, or body. Life is thus
the most essential condition of the body. _With life all portions of the
body are concerned, and toward its maintenance all the activities of the
body organization are directed_.
*The Nutrient Fluid in its Relations to the Cells.*--The maintenance of
life within the cells requires, as we have seen, that they be supplied
with water, food, and oxygen, and that they be relieved of such wastes as
they form. This double purpose is accomplished through the agency of an
internal nutrient fluid, a portion of which has already been referred to
as the lymph. Not only does this fluid supply the means for keeping the
cells alive, but, through the cells, it is also the means of preserving
the life of the body as a whole.
The cells, however, rapidly exhaust the nutrient fluid. They take from it
food and oxygen and they put into it their wastes. To prevent its becoming
unfit for supplying their needs, food and oxygen must be continually added
to this fluid, and waste materials must be continually removed. This is
not an easy task. As a matter of fact, the preparation, distribution, and
purification of the nutrient fluid requires the direct or indirect aid of
practically all parts of the body. It supplies for this reason a broad
basis for the division of labor on the part of the cells.
*Relation of the Body to its Environment.*--While life is directly
dependent upon the internal nutrient fluid, it is indirectly dependent
upon the physical surroundings of the body. Herein lies the need of the
_external_ organs--the feet and legs for moving about, the hands for
handling things, the eyes for directing movements, etc. That the great
needs of the body are supplied from its surroundings are facts of common
experience. Food, shelter, air, clothing, water, and the means of
protection are external to the body and form a part of its environment. In
making the things about him contribute to his needs, man encounters a
problem which taxes all his powers. Only by toil and hardship, "by the
sweat of his brow," has he been able to wrest from his surroundings the
means of his sustenance.
*The Main Physiological Problems.*--The study of the body is thus seen to
resolve itself naturally into the consideration of two main problems:
1. _That of maintaining in the body a nutrient fluid for the cells._
2. _That of bringing the body into such relations with its surroundings as
will enable it to secure materials for the nutrient fluid and satisfy its
other needs._
The first problem is _internal_ and includes the so-called vital
processes, known as digestion, circulation, respiration, and excretion.
The second problem is _external_, as it were, and includes the work of the
external organs--the organs of motion and of locomotion and the organs of
special sense. These problems are closely related, since they are the two
divisions of the one problem of maintaining life. Neither can be
considered independently of the other. In the chapter following is taken
up the first of these problems.
*Summary.*--The individual parts, or units, that form the body organization
are known as cells. These consist of minute but definitely arranged
portions of protoplasm and are held together by the intercellular
material. They build up the body and carry on its different activities.
The tissues are groups of like cells. By certain general activities the
cells maintain their existence in the tissues and by the exercise of
certain special activities they adapt the tissues to their purposes in the
body. The body, as a cell organization, has its activities directed under
normal conditions toward a single purpose--that of maintaining life. In the
accomplishment of this purpose a nutrient fluid is provided for the cells
and proper relations between the body and its surroundings are
established.
*Exercises.*--1. If a tissue be compared to a brick wall, to what do the
separate bricks correspond? To what the mortar between the bricks?
2. Draw an outline of a typical cell, locating and naming the main
divisions.
3. How do the cells enable the body to grow? Describe the process of
cell-division.
4. How does the general work of cells differ from their special work?
Define absorption, excretion, and assimilation as applied to the cells.
5. Compare the conditions surrounding a one-celled animal, living in
water, to the conditions surrounding the cells in the body.
6. What is meant by the term "environment"? How does man's environment
differ from that of a fish?
7. What is the necessity for a nutrient fluid in the body?
8. Why is the maintenance of life necessarily the chief aim of all the
activities of the body?
9. State the two main problems in the study of the body.
PRACTICAL WORK
*Observations.*--1. Make some scrapings from the inside of the cheek with a
dull knife and mix these with a little water on a glass slide. Place a
cover-glass on the same and examine with a compound microscope. The large
pale cells that can be seen in this way are a variety of epithelial cells.
2. Mount in water on a glass slide some thin slices of cartilage and
examine first with a low and then with a high power of microscope.
(Suitable slices may be cut, with a sharp razor, from the cartilage found
at the end of the rib of a young animal.) Note the small groups of cells
surrounded by, and imbedded in, the intercellular material.
3. Mount and examine with the microscope thin slices of elder pith,
potato, and the stems of growing plants. Make drawings of the cells thus
observed.
4. Examine with the microscope a small piece of the freshly sloughed off
epidermis of a frog's skin. Examine it first in its natural condition, and
then after soaking for an hour or two in a solution of carmine. Make
drawings.
5. Mount on a glass slide some of the scum found on stagnant water and
examine it with a compound microscope. Note the variety and relative size
of the different things moving about. The forms most frequently seen by
such an examination are one-celled plants. Many of these have the power of
motion.
6. Examine tissues of the body, such as nervous, muscular, and glandular
tissues, which have been suitably prepared and mounted for microscopic
study, using low and high powers of the microscope. Make drawings of the
cells in the different tissues thus observed.