which cellular activity is due may be destroyed.

In diseases due to parasites, the parasite produces a change in the
tissue in its immediate vicinity often so great as to result in the
death of the cells. The most general direct cause of lesions is toxic
or poisonous substances, either introduced from without or formed in
the body. In the case of the parasitic diseases the mere presence of
the parasite in the body produces little or no harm, the injury being
caused by poisons which it produces, and which act both locally in the
vicinity of the parasite and at a distance, being absorbed and
entering the blood stream. How certain of the poisonous substances act
is easy to see. Strong caustics act by coagulating the albumen, or by
the withdrawal of water from the cell. Other poisons act by forming
stable chemical compounds with certain of the cell constituents and
thereby preventing the usual chemical processes from taking place.
Death from the inhalation of illuminating gas is due to the carbon
monoxide contained in this, forming a firm chemical union with the
haemoglobin of the red corpuscles so that the function of these as
oxygen carriers is stopped.

In order that most poisons may act, it is essential that they enter
into the cell, and they cannot do this unless they are able to combine
chemically with certain of the cell constituents. To this is due the
selective action of many poisons. Morphine, for example, acts chiefly
on the cells of the brain; strychnine acts on the cells of the spinal
cord which excite motion and thus causes the characteristic muscular
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