|
Main
- books.jibble.org
My Books
- IRC Hacks
Misc. Articles
- Meaning of Jibble
- M4 Su Doku
- Computer Scrapbooking
- Setting up Java
- Bootable Java
- Cookies in Java
- Dynamic Graphs
- Social Shakespeare
External Links
- Paul Mutton
- Jibble Photo Gallery
- Jibble Forums
- Google Landmarks
- Jibble Shop
- Free Books
- Intershot Ltd
|
books.jibble.org
Previous Page
| Next Page
Page 4
Certain conditions are essential for the continued existence of living
matter. It must be surrounded by a fluid or semi-fluid medium in order
that there may be easy interchange with the environment. It must
constantly receive from the outside a supply of energy in the form of
food, and substances formed as the result of the intracellular
chemical activity must be removed. In the case of many animals it
seems as though the necessity of a fluid environment for living matter
did not apply, for the superficial cells of the skin have no fluid
around them; these cells, however, are dead, and serve merely a
mechanical or protective purpose. All the living cells of the skin and
all the cells beneath this have fluid around them.
Living matter occurs always in the form of small masses called
"cells," which are the living units. The cells vary in form, structure
and size, some being so large that they can be seen with the naked
eye, while others are so small that they cannot be distinctly seen
with the highest power of the microscope. The living thing or organism
may be composed of a single cell or, in the case of the higher animals
and plants, may be formed of great numbers of cells, those of a
similar character being combined in masses to form organs such as the
liver and brain.
In each cell there is a differentiated area constituting a special
structure, the nucleus, which contains a peculiar material called
"chromatin." The nucleus has chiefly to do with the multiplication of
the cell and contains the factors which determine heredity. The mass
outside of the nucleus is termed "cytoplasm," and this may be
homogeneous in appearance or may contain granules. On the outside
there is a more or less definite cell membrane. It is generally
believed that the cell material has a semi-fluid or gelatinous
consistency and is contained within an intracellular meshwork. It is
an extraordinarily complex mass, whether regarded from a chemical or
physical point of view. (Fig. 1.)
[Illustration: FIG. 1.--DIAGRAM OF CELL. 1. Cell membrane. 2. Cell
substance or cytoplasm. 3. Nucleus. 4. Nuclear membrane.
5. Nucleolus.]
A simple conception of health and disease can be arrived at by the
study of these conditions in a unicellular animal directly under a
microscope, the animal being placed on a glass slide. For this purpose
a small organism called "Amoeba" (Fig. 2), which is commonly present
in freshwater ponds, may be used. This appears as a small mass,
seemingly of gelatinous consistency with a clear outline, the exterior
part homogeneous, the interior granular. The nucleus, which is seen
with difficulty, appears as a small vesicle in the interior. Many
amoeb� show also in the interior a small clear space, the contractile
vesicle which alternately contracts and expands, through which action
the movement of the intracellular fluid is facilitated and waste
products removed. The interior granules often change their position,
showing that there is motion within the mass. The amoeba slowly moves
along the surface of the glass by the extension of blunt processes
formed from the clear outer portion which adhere to the surface and
into which the interior granular mass flows. This movement does not
take place by chance, but in definite directions, and may be
influenced. The amoeba will move towards certain substances which may
be placed in the fluid around it and away from others. In the water in
which the amoeb� live there are usually other organisms, particularly
bacteria, on which they feed. When such a bacterium comes in contact
with an amoeba, it is taken into its body by becoming enclosed in
processes which the amoeba sends out. The enclosed organism then lies
in a small clear space in the amoeba, surrounded by fluid which has
been shown to differ in its chemical reaction from the general fluid
of the interior. This clear space, which may form at any point in the
body, corresponds to a stomach in a higher animal and the fluid within
it to the digestive fluid or gastric juice. After a time the enclosed
organism disappears, it has undergone solution and is assimilated;
that is, the substances of which its body was composed have been
broken up, the molecules rearranged, and a part has been converted
into the substance of the amoeba. If minute insoluble substances, such
as particles of carmine, are placed in the water, these may also be
taken up by the amoeba; but they undergo no change, and after a time
they are cast out. Under the microscope only the gross vital
phenomena, motion of the mass, motion within the mass, the reception
and disintegration of food particles, and the discharge of inert
substances can be observed. The varied and active chemical changes
which are taking place cannot be observed.
[Illustration: FIG. 2.--AMOEBA. 1. Nucleus. 2. Contractile vesicle.
3. Nutritive vacuole containing a bacillus.]
Previous Page
| Next Page
|
|