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Page 31
If we regard the living things on earth from the narrow point of view
as to whether they are necessary or useless or hostile to man, the
protozoa must be regarded as about the least useful members of the
biological society. It is very possible that such a conclusion is due
to ignorance; so closely are all living things united, so dependent is
one form of cell activity upon other forms that it is impossible to
foretell the result of the removal of a link. The protozoa do not seem
to be as necessary for the life of man as are the bacteria; they
produce many of the diseases of man, many of the diseases of animals
on which man depends for food; they cause great destruction in plant
life, and in the soil they feed upon the useful bacteria. It is well
to remember, however, that fifty years ago several of the organs of
the body whose activity we now recognize as furnishing substances
necessary for life were regarded as useless members and, since they
became the seat of tumors, as dangerous members of the body. The only
organ which now seems to come into such a class is the vermiform
appendix, and its lowly position among organs is due merely to an
unhappy accident of development.
The class of organisms known as the filterable viruses or the
ultra-microscopic or the invisible organisms have a special interest
in many ways. The limitation in the power of the microscope for the
study of minute objects is due not to a defect in the instrument but
to the length of the wave of light. It is impossible to see clearly
under the microscope using white light, objects which are smaller in
diameter than the length of the wave which gives a limit of 0.5�. or
1/125,000 of an inch. By using waves of shorter length, as the
ultra-violet light, objects of 0.1�. or 1/250000 of an inch can be
seen; but as these methods depend upon photography for the
demonstration of the object the study is difficult. The presence of
objects still smaller than 0.1 m. can be detected in a fluid by the
use of the dark field illumination and the ultra-microscope, the
principle of which is the direction of a powerful oblique ray of light
into the field of the microscope. The objects are not visible as such,
but the dispersion of the light by their presence is seen.
The demonstration that infectious diseases were produced by organisms
so small as to be beyond demonstration with the best microscopes was
made possible by showing, that some fluid from a diseased animal was
infectious; and capable of producing the disease when inoculated into
a susceptible animal. The fluid was then filtered through porcelain
filters which were known to hold back all objects of the size of the
smallest bacteria and the disease produced by inoculating with the
clear filtrate. There are a number of such filters of different
degrees of porosity manufactured, and they are often used to procure
pure water for drinking, for which use they are more or less,
generally however, less efficacious. The filter has the form of a
hollow cylinder and the liquid to be filtered is forced through it
under pressure. For domestic use the filter is attached by its open
end to the water tap and the pressure from the mains forces the water
through it. In laboratory uses, denser filters of smaller diameters
are used, and the filter is surrounded by the fluid to be tested. The
open end of the filter passes into a vessel from which the air is
exhausted and filtration takes place from without inward. The test of
the effectiveness of the filter is made by adding to the filtering
fluid some very minute and easily recognizable bacteria and testing
the filtrate for their presence. These filters have been studied
microscopically by grinding very thin sections and measuring the
diameter of the spaces in the material. These are very numerous, and
from 1/25000 to 1/1000 of an inch in diameter, spaces which would
allow bacteria to pass through, but they are held back by the very
fine openings between the spaces and by the tortuosity of the
intercommunications. When the coarser of such filters have been long
in domestic service in filtering drinking water, bacteria may grow in
and through them giving greater bacterial content to the supposed
bacteria-free filtrate than in the filtering water.
That an animal disease was due to such a minute and filterable
organism was first shown by Loeffler in 1898 for the foot and mouth
disease of cattle. This is one of the most infectious and easily
communicable diseases. The lesions of the disease take the form of
blisters which form on the lips and feet and in the mouths of cattle,
and inoculation with minute quantities of the fluid in the blisters
produces the disease. Loeffler filtered the fluid through porcelain
filters, hoping to obtain a material which inoculated into other
cattle would render them immune, and to his surprise found that the
typical disease was produced by inoculating with the filtrate.
Naturally the first idea was that the disease was caused by some
soluble poison and not by a living organism, but this was disproved in
a number of ways. The most powerful poison known is obtained from
cultures of the tetanus bacillus of which 0.000,000,1 of a gram (one
gram is 15.43 grains) kills a mouse, or one gram kills ten million
mice. Loeffler found that 1/30 gram of the contents of the vesicles
killed a calf of two hundred kilograms weight, and assuming that the
essential poison was present in the fluid in one part to five hundred
it would be several hundred times more powerful than the tetanus
poison. Further, the disease produced by inoculation of the filtrate
was itself inoculable and could be transmitted from animal to animal.
It was also found that when the virus was filtered several times it
ceased to be inoculable, showing that each time the fluid was passed
through the filter some of the minute organisms contained in it were
held back.
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