388 ON THE RELATIONS OF MICRO-ORGANISMS TO DISEASE 
human species, as has been lately illustrated by the so-called ‘ woolsorters’ 
disease’, in the North of England. The Bacillus anthracis is a large form of 
bacterium, as is shown in the accompanying woodcut, a. It is there repre- 
sented magnified 700 diameters, along with the outlines of some red blood- 
corpuscles of a mouse, and the rods of which it is composed are seen to be in 
diameter nearly one fourth of that of the red corpuscles. Koch’s method of 
staining sections shows in the most beautiful manner that these bacilli are not 
only present in the spleen and some other organs, but that they people the 
blood in the minute vessels of all parts. Koch has thus added to our conviction 
that the bacillus is the cause of the symptoms, seeing that, as he remarks, it is 
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impossible to suppose that an organism can develop in such enormous numbers 
at the expense of the vital fluid without exerting a serious influence upon the 
system. . 
But the most striking and important results of Koch’s methods of investiga- 
tion are those which relate to organisms of much smaller dimensions. He found 
that, if putrid liquid is injected under the skin of a mouse, the animal may 
die in the course of a short time, as the result of the chemically toxic effects of 
the products of putrefaction absorbed into the circulation ; but, if it survive 
this primary disorder, it may succumb in the course of about two days to blood 
disease. If the point of a lancet be dipped into the blood of the heart of a mouse 
which has died in this way, and a scratch be made in the skin of a healthy mouse 
with the envenomed instrument, the second mouse dies with similar symptoms 
to those of the first, the poison being absolutely certain in its virulent operation ; 
and the same thing may be continued indefinitely through any series of animals. 
If now sections of the tissues be made and stained and examined by Koch’s pro- 
cedures, it is found that the entire blood of the diseased animal is peopled with 
bacteria, resembling those of the Bacillus anthracis in the enormous multitudes in 
which they are produced, and also in their rod-like form, but differing from them 
in being exquisitely minute and delicate, as is shown in } (drawn on the same 
