ON THE RELATIONS OF MICRO-ORGANISMS TO DISEASE 389 
scale as a, as is indicated by the accompanying outlines of red corpuscles), 
where it is seen that the diameter can only be represented by a slender streak 
not one-eighth of the diameter of the Bacillus anthracis, and such as, before 
the introduction of Koch’s method, would have escaped notice altogether. 
Now, this disease is totally distinct from pyaemia, being not accompanied with 
multiple abscesses or embolism; and thus it has been shown by Koch that 
septicaemia may exist as a deadly blood disease, caused by the development 
of micro-organisms, equally distinct from pyaemia and from the chemically 
toxic effects of septie products. 
On some occasions, as the result of the introduction of putrid fluid under 
the mouse’s skin, Koch found, besides septicaemia, a local affection of the seat 
of inoculation, in the form of spreading gangrene ; and, on investigating the part, 
he discovered in it, exactly corresponding with the extent of the local affection, 
another organism very differently formed from that of the septicaemia, viz. a 
micrococcus, consisting of minute spherical granules arranged in linear series, 
like strings of exquisitely minute beads, as represented at c, which is magnified 
hike a and 6 700 diameters. Believing that this locally developing organism 
must be the cause of the gangrene, he tried to separate it from the bacillus of 
the septicaemia, and succeeded through an accidental observation of great 
interest. Having till that time employed the house mouse in his experiments, 
he happened to try the inoculation of a field mouse. This animal, though so 
closely allied, proved not susceptible of the septicaemia. The bacillus of that 
disease was unable to grow in the blood of the field mouse; but the micro- 
coccus of the gangrene could develop among its tissues. The new organism 
was thus obtained in an isolated form, and, when now inoculated into the house 
mouse, produced in that animal gangrene pure and simple, extending for an 
indefinite period among its tissues. 
Thus the animal body, which had previously been an obscure field of labour 
in this department, in which the pathologist did little more than grope in the 
dark, was converted by Koch into a pure cultivating apparatus, in which the 
growth and effects of the micro-organisms of various infective diseases could be 
studied with the utmost simplicity and precision. 
One more example I must take from Koch’s work. On one occasion, as 
the result of inoculating putrid liquid into a rabbit, he observed a spreading 
