spasm. The poisonous substances produced by bacteria, as in the case
of diphtheria, act on certain of the organs only. Different animal
species owe their immunity to certain poisons to their cells being so
constituted that a poison cannot gain entrance into them; pigeons, for
example, cannot be poisoned by morphia. Individual variations play an
important part also; thus, shellfish are poisonous for certain
individuals and not so for others. Owing to the variability of living
structures a substance may be poisonous at one time and not at
another, as the following example shows. A man, very fond of crab
meat, was once violently poisoned after eating crabs, being at that
time seemingly in his usual state of health, and no illness resulted
in others who had partaken of the same crabs. Two months later a
hearty meal of crabs produced no ill result. There are also
individuals so constituted that so simple a food as the egg is for
them an active poison.

The lesions produced by the action of injurious conditions are usually
so distinctive in situation and character that by the examination of
the body after death the cause of death can be ascertained. The
lesions of diseases may be very obvious to the naked eye, or in other
cases only the most careful microscopic examination can detect even
the presence of alterations. In the case of poisons the capacity of
the cell for adaptation to unusual conditions is of great importance.
It is probable that certain changes take place within the cells, owing
to which the function can be continued in spite of the unusual
conditions which the presence of the poison brings about. It is in
this way that the habitual use of such poisons as morphine, alcohol
and tobacco, to speak only of those best known, is tolerated. The cell
life can become so accustomed to the presence of poisons that the cell
activities may suffer in their absence.