A Practical Physiology | Page 5

in appearance, and on the threads of the meshwork may be
one or more enlargements, called nucleoli. In some cases the
protoplasm at the circumference is so modified as to give the
appearance of a limiting membrane called the cell wall. In brief, then, a
cell is a mass of nucleated protoplasm; the nucleus may have a
nucleolus, and the cell may be limited by a cell wall. Every tissue of the
human body is formed through the agency of protoplasmic cells,
although in most cases the changes they undergo are so great that little
evidence remains of their existence.
There are some organisms lower down in the scale, whose whole
activity is confined within the narrow limits of a single cell. Thus, the
amoeba begins its life as a cell split off from its parent. This divides in
its turn, and each half is a complete amoeba. When we come a little
higher than the amoeba, we find organisms which consist of several
cells, and a specialization of function begins to appear. As we ascend in
the animal scale, specialization of structure and of function is found
continually advancing, and the various kinds of cells are grouped

together into colonies or organs.
12. Cells and the Human Organism. If the body be studied in its
development, it is found to originate from a single mass of nucleated
protoplasm, a single cell with a nucleus and nucleolus. From this
original cell, by growth and development, the body, with all its various
tissues, is built up. Many fully formed organs, like the liver, consist
chiefly of cells. Again, the cells are modified to form fibers, such as
tendon, muscle, and nerve. Later on, we shall see the white blood
corpuscles exhibit all the characters of the amoeba (Fig. 2). Even such
dense structures as bone, cartilage, and the teeth are formed from cells.
[Illustration: Fig. 2.--Amoeboid Movement of a Human White Blood
Corpuscle. (Showing various phases of movement.)]
In short, cells may be regarded as the histological units of animal
structures; by the combination, association, and modification of these
the body is built up. Of the real nature of the changes going on within
the living protoplasm, the process of building up lifeless material into
living structures, and the process of breaking down by which waste is
produced, we know absolutely nothing. Could we learn that, perhaps
we should know the secret of life.
13. Kinds of Cells. Cells vary greatly in size, some of the smallest
being only 1/3500 an inch or less in diameter. They also vary greatly in
form, as may be seen in Figs. 3 and 5. The typical cell is usually
globular in form, other shapes being the result of pressure or of similar
modifying influences. The globular, as well as the large, flat cells, are
well shown in a drop of saliva. Then there are the columnar cells, found
in various parts of the intestines, in which they are closely arranged
side by side. These cells sometimes have on the free surface delicate
prolongations called cilia. Under the microscope they resemble a wave,
as when the wind blows over a field of grain (Fig. 5). There are besides
cells known as _spindle, stellate, squamous_ or pavement, and various
other names suggested by their shapes. Cells are also described as to
their contents. Thus fat and pigment cells are alluded to in succeeding
sections. Again, they may be described as to their functions or location
or the tissue in which they are found, as epithelial cells, blood cells
(corpuscles, Figs. 2 and 66), nerve cells (Fig. 4), and
_connective-tissue_ cells.
14. Vital Properties of Cells. Each cell has a life of its own. It manifests

its vital properties in that it is born, grows, multiplies, decays, and at
last dies.[3] During its life it assimilates food, works, rests, and is
capable of spontaneous motion and frequently of locomotion. The cell
can secrete and excrete substance, and, in brief, presents nearly all the
phenomena of a human being.
Cells are produced only from cells by a process of self-division,
consisting of a cleavage of the whole cell into parts, each of which
becomes a separate and independent organism. Cells rapidly increase in
size up to a certain definite point which they maintain during adult life.
A most interesting quality of cell life is motion, a beautiful form of
which is found in ciliated epithelium. Cells may move actively and
passively. In the blood the cells are swept along by the current, but the
white corpuscles, seem able to make their way actively through the
tissues, as if guided by some sort of instinct.
[Illustration: Fig. 3.--Various Forms of Cells.
A, columnar cells found lining various parts of the intestines (called
_columnar epithelium_); B, cells of a fusiform or spindle