142 Proceedings of the Royal Physical Socrety. 
I do not attempt to trace the steps by which this im- 
portant discovery was made, but you can imagine what an 
attractive and almost startling discovery it was to find that 
there was in the body a standing army of this kind, its 
members flying about in the blood stream, ready to pounce 
upon and destroy any noxious matter or active microbe, and 
having depots scattered all up and down throughout the 
body, most numerous where the danger is greatest. 
This phagocytic action of the leucocytes is so marked and 
so capable of demonstration in some cases, that some of the 
more ardent upholders of it seek to explain the preventive 
and curative effects of serum to the influence it exerts upon 
the leucocytes. They argue in this wise. The phagocytic 
action of the cells is assumed, and the important influence 
of the blood-serum is also acknowledged, for in some cases 
it is too striking to be denied. It is suggested that the 
constitution of the blood-serum depends a good deal upon 
the influence the leucocytes exercise, that, in fact, they 
elaborate substances which are required for the defence of. 
the body, and that they give up these into the serum. It is 
contended that the action of the antitoxic serums is to 
stimulate the production of these defensive substances, and 
that in this way immunity is effected. 
The subject is very complex, and I think it extremely 
improbable that any rigid explanation of the varied pheno- 
mena is applicable. 
As to certain facts there can be no question, and as to 
certain effects there is none. Putting it in relief, the facts 
are, that certain toxines are fatal, that by various processes 
animals can be rendered proof against this fatal action, and 
that the serum of such animals renders other animals also 
proof against them. This is evidently and necessarily due 
to important modifications in the blood-serum, but what 
these precise modifications are have not yet been fully shown. 
That the leucocytes and the tissue in which they originate 
play an important part in the production of the effect is at 
least probable, since it has been found that it is impossible 
to render an animal refractory to at least certain toxines, as 
tetanus, if its spleen has been removed. While this is so, 
