ich
diffuse for a distance around the bacteria. This and similar facts have
suggested that some toxins are only produced in the living body. A
considerable amount of work has been done in connexion with this subject,
and many observers have found that fluids taken from the living body in
which the organisms have been growing, contain toxic substances, to which
the name of _aggressins_ has been applied. Fluid containing these
aggressins greatly increases the toxic effect of the corresponding
bacteria, and may produce death at an earlier stage than ever occurs with
the bacteria alone. They also appear to have in certain cases a paralysing
action on the cells which act as phagocytes. The [v.03 p.0174] work on this
subject is highly suggestive, and opens up new possibilities with regard to
the investigation of bacterial action within the body. Not only are the
general symptoms of poisoning in bacterial disease due to toxic substances,
but also the tissue changes, many of them of inflammatory nature, in the
neighbourhood of the bacteria. Thus, to mention examples, diphtheria toxin
produces inflammatory oedema which may be followed by necrosis; dead
tubercle bacilli give rise to a tubercle-like nodule, &c. Furthermore, a
bacillus may give rise to more than one toxic body, either as stages in one
process of change or as distinct products. Thus paralysis following
diphtheria is in all probability due to a different toxin from that which
causes the acute symptoms of poisoning or possibly to a modification of it
sometimes formed in specially large amount. It is interesting to note that
in the case of the closely analogous example of snake venoms, there may be
separated from a single venom a number of toxic bodies which have a
selective action on different animal tissues.

[Sidenote: Nature of toxins.]

Regarding the chemical nature of toxins less is known than regarding their
physiological action. Though an enormous amount of work has been done on
the subject, no important bacterial toxin has as yet been obtained in a
pure condition, and, though many of them are probably of proteid nature,
even this cannot be asserted with absolute certainty. Brieger, in his
earlier work, found that alkaloids were formed by bacteria in a variety of
conditions, and that some of them were poisonous. These alkaloids he called
_ptomaines_. The methods used in the investigations were, however, open to
objection, and it is now recognized that although