ever with increased protein metabolism, attended with disturbances of the
circulatory and respiratory Systems. Nervous symptoms, somnolence, coma,
spasms, convulsions and paralysis are of common occurrence. All such
phenomena, however, are likewise due to the disturbance of the molecular
constitution of living cells. Alterations in metabolism are found to be
associated with some of these, but with others no corresponding physical
change can be demonstrated. The action of toxins on various glands,
producing diminished or increased functional activity, has a close analogy
to that of certain drugs. In short, if we place aside the outstanding
exception of tumour growth, we may say that practically all the important
phenomena met with in disease may be experimentally produced by the
injection of bacteria or of their toxins.

[Sidenote: Susceptibility.]

The result of the entrance of a virulent bacterium into the tissues of an
animal is not a disease with hard and fast characters, but varies greatly
with circumstances. With regard to the subject of infection the chief
factor is susceptibility; with regard to the bacterium virulence is
all-important. Susceptibility, as is well recognized, varies much under
natural conditions in different species, in different races of the same
species, and amongst individuals of the same race. It also varies with the
period of life, young subjects being more susceptible to certain diseases,
_e.g._ diphtheria, than adults. Further, there is the very important factor
of acquired susceptibility. It has been experimentally shown that
conditions such as fatigue, starvation, exposure to cold, &c., lower the
general resisting powers and increase the susceptibility to bacterial
infection. So also the local powers of resistance may be lowered by injury
or depressed vitality. In this way conditions formerly believed to be the
causes of disease are now recognized as playing their part in predisposing
to the action of the true causal agent, viz. the bacterium. In health the
blood and internal tissues are bacterium-free; after death they offer a
most suitable pabulum for various bacteria; but between these two extremes
lie states of varying liability to infection. It is also probable that in a
state of health organisms do gain entrance to the blood from time to time
and are rapidly killed off. The circumstances which alter the virulence of
bacteria will be referred to again in connexion with immunity, but i