136 Proceedings of the Royal Physical Society. 
A satisfactory method of attenuation having been dis- 
covered, the fact that the horse was less susceptible to the 
diphtheria poison than most of the other animals tried 
indicated the direction of least resistance, and this animal was 
rendered immune by injections of the attenuated virus in the 
way I have indicated. It was then found that the blood- 
serum of the immunised horse had a protective effect, and 
even a curative one, when injected into other animals. At 
this stage it was deemed safe to try the serum in the human 
being, and as all of you doubtless know, this method has 
been extensively used, has been found of the greatest value, 
and has already been the means of saving very many human 
lives. 
In this instance are illustrated two facts—one, the attenua- 
tion of a virus by mixing it with a chemical substance, this 
attenuation rendering it suitable for repeated inoculations, 
and these producing a condition of immunity in the animal 
inoculated with them; the other, that the possession of a 
degree of immunity-determined the selection of the animal 
to be rendered wholly immune, So that in this the leading 
of nature is followed and utilised. 
If we consider the mode of action of this antidiphtheritic 
serum, one of the views which is held regarding it is that by 
its admixture with the toxin, either within or without the 
body, the poison is destroyed by contact, or that the union of 
the two produces an inoffensive substance. But this theory 
is hardly tenable, for under certain circumstances the toxic 
effect is produced notwithstanding its admixture with anti- 
toxic serum. This was the effect produced in some guinea- 
pigs which were at the time in perfect health, but had some 
time previously been inoculated against another disease. 
This illustrates the curious fact that these guinea-pigs must 
have had their constitution (to use the word in its popular 
sense) modified by this antecedent vaccination, and that 
modification, which in no way affected their health, rendered 
them even more than usually susceptible to the diphtheria 
toxin. The changes in the blood must thus be of an 
extremely subtle kind, and you can understand how very 
complex the conditions may be which have to be met. The 
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