coagulate at a much lower temperature than the white of the egg (as
the myosin, one of the albumens of the muscle which coagulates at 115°
F., egg white coagulating at 158° F.), and in addition to such
coagulation or without it the ferments within the cell and to the
action of 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
hæmoglobin 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