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Page 26
Knowledge of the minute organisms was slowly accumulating. The first
questions to be determined were as to their nature and origin. How
were they produced? Did they come from bodies of the same sort
according to the general laws governing the production of living
things, or did they arise spontaneously? a question which could not be
solved by speculation but by experiment. The first experiments, by
Needham, 1745, pointed to the spontaneous origin of the organisms. He
enclosed various substances in carefully sealed watch crystals from
which the air was excluded, and found that animalculi appeared in the
substance, and argued from this that they developed spontaneously. In
1769, Spallanzani, a skilled experimental physiologist, in a brilliant
series of experiments showed the imperfect character of Needham's work
and the fallacy of his conclusions. Spallanzani placed fluids, which
easily became putrid, in glass tubes, which he then hermetically
sealed and boiled. He found that the fluid remained clear and
unchanged; if, however, he broke the sealed point of such a tube and
allowed the air to enter, putrefaction, or in some cases fermentation,
of the contents took place. He concluded that boiling the substances
destroyed the living germs which they contained, the sealed tubes
prevented the air from entering, and when putrefaction or fermentation
of the contents took place the organisms to which this was due, being
contained in the air, entered from without. Objection was made to the
conclusions of Spallanzani that heating the air in the closed tubes so
changed its character as to prevent development of organisms in the
contents. This objection was finally set aside by Pasteur, who showed
that it was not necessary to seal the end of the tube before boiling,
but it could be closed by a plug of cotton wool, which mechanically
removed the organisms from the air which entered the tube, or if the
tube were bent in the shape of a _U_ and the end left open,
organisms from the air could not pass into the tube against gravity
when air movement within the tube was prevented by bending. The
possibility of spontaneous generation cannot be denied, but that it
takes place is against all human experience.
It was not possible to attain any considerable knowledge of the
bacteria discovered by Loewenhoeck until more perfect instruments for
studying them were devised. Lenses for studying objects were used in
remote antiquity, but the compound microscope in which the image made
by the lens is further magnified was not discovered until 1605, and
when first made was so imperfect that the best simple lenses gave
clearer definition. With the betterment of the microscope, increasing
the magnifying power and the sharpness of the image of the object
seen, it became possible to classify the minute organisms according to
size and form and to study the separate species. The microscope has
now reached such a degree of perfection that objects smaller than one
one hundred thousandth of an inch in diameter can be clearly seen and
photographed.
Great impetus was given to the biological investigation of disease by
the discoveries which led to the formulation of the cell theory in
1840 and the brilliant work of Pasteur on fermentation,[1] but it was
not until 1878 that it was definitely proved that a disease of cattle
called anthrax was due to a species of bacteria. What should be
regarded as such proof had been formulated by Henle in 1840. To prove
that a certain sort of organism when found associated with a disease
is the cause of the disease, three things are necessary:
1. The organism must always be found in the diseased animal and
associated with the changes produced by the disease.
2. The organism so found must be grown outside of the body in what is
termed pure cultures, that is, not associated with any other
organisms, and for so long a time with constant transfers or new
seedings that there can be no admixture of other products of the
disease in the material in which it is grown.
3. The disease must be produced by inoculating a susceptible animal
with a small portion of such a culture, and the organism shown in
relation to the lesions so produced.
It is worth while to devote some attention to the disease anthrax.
This occupies a unique position, in that it was the first of the
infectious diseases to be scientifically investigated. In this
investigation one fact after another was discovered and confirmed;
some of these facts seemed to give clearer conceptions of the disease,
others served to make it more obscure; new questions arose with each
extension of knowledge; in the course of the work new methods of
investigation were discovered; the sides of the arch were slowly and
painfully erected by the work of many men, and finally one man placed
the keystone and anthrax was for a long time the best known of
diseases. Men whose reputation is now worldwide first became known by
their work in this disease. It was a favorable disease for
investigation, being a disease primarily of cattle, but occasionally
appearing in man, and the susceptibility of laboratory animals made
possible experimental study.
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