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perhaps as free motile nuclei, such as are noticeable in the majority of specimens and
distinguishable from the free nuclei of healthy blood only by a sort of purpose in
their movements, but more probably as in a quiescent form so that they are detectable
only on aniline staining.* Whether naturally the organism enters the system in the
spirilloid forms, or as a micrococcus, or as an unrecognizable germ, we cannot say, and
we are equally without guiding evidence as to whether its entry is effected by exercise
of its eel-like activity or by its use of a special boring organ too minute for detection
with ordinary microscopical powers. The size of spirilloids is not sufficiently great to
exclude the possibility of their passing through the walls of normal capillaries, as do
the leucocytes, but the passage may be to an extent forced, for we have evidence that
the blunt extremity or " head " must be armed with some mechanism, if only a sucker,
to enable it to obtain a grip on the red corpuscle; the disintegration of the latter
would lead us to infer that the arrangement is something more formidable than a sucker.
"We know that the parasites, small though they be, can exercise a great deal of strength
at the expense of the corpuscles. The amount of force expended is often greater than
that which is exerted in adhesion between two corpuscles in a rouleau; it is sufficient
to enable the parasites to drag the corpuscles about, shake them, distort them into
various shapes, and it suffices to enable one parasite to drag two corpuscles about with
ease and rapidity, also to shake a long rouleau, but not to detach it from the mesh-
work to which it belongs. This strength is ordinarily exercised in locomotion, in
traction, in changes of form, and in pushing a way among the corpuscles. The energy,
strength, and perseverance shown by the parasites in making their way through an
aggregated mass of red corpuscles should more than suffice to procure them an entry
into living capillaries. The parasites in their spirilloid form, much more so in their
possible germ phase of development, would find no difficulty, under the ordinary life
conditions of mules, in obtaining access to the mucous membranes of the alimentary
and respiratory systems; entry via the skin is not impossible especially in the presence
of open wounds, blistered surfaces, &c, but we have evidence against its ordinary
occurrence. The air, water, and food must therefore be looked on with suspicion as
vehicles for conveyance of the organism which generates relapsing fever into the
system of the victim.
15. But not only is it necessary that the organism shall gain access to the system,
it is also essential that the individual invaded be susceptible to attack in order that the
parasite may obtain a fail footing from which to extend its ravages. My inoculations
of the bullock have thus far been valuable, in that they have shown that local irritant
action by the parasite need not necessarily be followed by systemic infection. That
the opposite is not the case is shown by my inoculation of the pony in which both local
and general symptoms were the result. In many diseases individual peculiarities
have a marked effect on the " taking" or rejection of the specific virus, but in
equine patients my administrations of relapsing fever parasites have been invariably
followed by success in communication of the disease. Animals in fat condition have
succumbed as rapidly (perhaps more so) as the weak and emaciated ; old and young, male
and female, horse and mule, have become affected. Thus in this disease, as in anthrax,
but to a less degree, predisposing causes sink into insignificance from a practical point
of view in comparison with the determining influences, viz., those which have a direct
influence in conveyance of the pathogenic organisms into the system, for we can alter
the latter, but the former are beyond our control to a great degree, or, however much
we may modify them, we cannot render the system absolutely insusceptible to invasion
by any means we are yet acquainted with. Yet we cannot altogether afford to ignore
predispositions on the part of the animal, and so shall have something to say about
age, color, breed, and conformation after we have drawn attention to a few other
matters to be taken into consideration in studying the causes of disorder. It is evident
that the same cause operating on different individuals does not necessarily produce the
same effect. Thus many animals exposed to a specific influence may resist it, whereas
others succumb. Therefore we must not argue that, because a supposed cause is not
in every case followed by an attack, it is not a true cause, nor must we, on the other
hand, consider that, because a supposed cause fails to generate disease, the animal is
* In his report on " Surra" Dr. Evans says :—" The blood parasites do not disappear from the blood absolutely when
it becomes difficult to find one in a drop of blood ; when one brood or generation dies there are ova or spores left for the
development of another brood.''