760 SPONTANEOUS CONGESTION AND INFECTIOUS DISEASES. 
for themselves and apply the nourishment which they receive to their own 
purposes. Prom such innocent productions as corns and warts, they are 
gradations to the serious tumours, which, by their mere size and the 
mechanical obstruction they cause, destroy the organism out of which they 
are developed ; while, finally, in those terrible structures, known as cancers, 
the abnormal growth has acquired powers of reproduction and multiplication, 
and is only morphologically distinguishable from the parasitic worm, the 
life of which is neither more or less closely bound up with that of the 
infested organism. If there were a kind of diseased structure, the histo- 
logical elements of which were capable of maintaining a separate and 
independent existence out of the body, it seems to me that the shadowy 
boundary between morbid growth and Xenogenesis would be effaced. And 
I am inclined to think that the progress of discovery has almost brought us 
to this point already. 
I have been favoured by Mr. Simon with an early copy of the last 
published of the valuable ‘ Reports on the Public Health/ which, in his 
capacity of their medical officer, he annually presents to the Lords of the 
Privy Council. The appendix to this report contains an introductory essay 
“ On the Intimate Pathology of Contagion,” by Dr. Burdon Sanderson, 
which is one of the clearest, most comprehensive, and well-reasoned discus- 
sions of a great question which has come under my notice for a long time. 
I refer you to it for details and for the authorities for the statements I am 
about to make. You are familiar with what happens in vaccination. A 
minute cut is made in the skin, and an infinitesimal quantity of vaccine 
matter is inserted into the wound. Within a certain time a vesicle appears 
in the place of the wound, and the fluid which distends this vesicle is vaccine 
matter, in quantity a hundred or a thousand fold that which was originally 
inserted. Now what has taken place in the course of this operation? Has 
the vaccine matter by its irritative property produced a mere blister, the 
fluid of which has the same irritative property ? Or does the vaccine matter 
contain living particles, which have grown and multiplied where they have 
been planted ? The observations of M. Chauveau, extended and confirmed 
by Dr. Sanderson himself, appear to leave no doubt upon this head. 
Experiments, similar in principle to those of Helmholtz on fermentation 
and putrefaction, have proved that the active element in the vaccine lymph 
is non -diffusible, and consists of minute particles not exceeding 1- 20,000th 
of an inch in diameter, which are made visible in the lymph by the micro- 
scope. Similar experiments have proved that two of the most destructive 
of epizootic diseases, sheep-pox and glanders, are also dependent for their 
existence and their propagation upon extremely small living solid particles, 
to which the title of “ microzymes ” is applied. An animal suffering under 
either of these terrible diseases is a source of infection and contagion to 
others, for precisely the same reason as a tub of fermenting beer is capable 
of propagating its fermentation by “ infection ” or “ contagion/’ to fresh 
wort. In both cases it is the solid living particles which are efficient ; the 
liquid in which they float, and at the expense of which they live, being alto- 
gether passive. Now arises the question, are these microzymes the results 
of homogenesis, or of xenogenesis ; are they capable, like the torulae of yeast, 
of arising only by the development of pre-existing germs ; or may they be, 
like the constituents of a nut-gall, the results of L a modification and indivi- 
dualisation of the tissues of the body in which they are found, resulting Irom 
the operation of certain conditions ? Are they the parasites in the zoologi- 
cal sense, or are they merely what Virchow has called “heterologous 
growths ?” It is obvious that this question has the most profound impor- 
tance, whether we look at it from a practical or from a theoretical point of 
view. A parasite may be stamped out by destroying its germs, but a patho- 
