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Page 12
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
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.
_Repair_ of the injuries which the body receives is effected in a
variety of ways. We do not know how intracellular repair takes place,
but most probably the cells get rid of the injured areas either by
ejecting them, or chemical changes are produced in the altered cell
substance breaking up and recombining the molecules. When single cells
are destroyed, the loss is made good by new formation of cells, the
cell loss stimulating the formative activity of the cells in the
vicinity. The body maintains a cell and tissue equilibrium, and a loss
is in most cases repaired. The blood fluid lost in a h�morrhage is
quickly restored by a withdrawal of the fluid from the tissues into
the blood, but the cells lost are restored by new formation of cells
in the blood-forming organs. The blood cells are all formed in bone
marrow and in the lymph nodes, and not from the cells which circulate
in the blood, and the stimulus to new cell formation which the loss of
blood brings about affects this remote tissue.
In general, repair takes place most easily in tissues of a simple
character, and where there is the least differentiation of cell
structure for the purposes of function. A high degree of function in
which the cell produces material of a complex character necessitates a
complex chemical apparatus to carry this out, and a complicated
mechanism is formed less easily than a simple one. In certain tissues
the cells have become so highly differentiated that all formative
activity is lost. Such is the case in the nerve cells of the brain and
spinal cord, a loss in which tissue is never repaired by the formation
of new cells; and in the muscles the same is true. The least
differentiation is seen in those cells which serve the purpose of
mechanical protection only, as the cells of the skin, and in these the
formative activity is very great. Not only must the usual loss be
supplied, but we are all conscious of slight injuries of the surface
which are quickly repaired.
Repair, other things being equal, takes place more easily in the young
than in the old. New formation of cells goes on with great rapidity in
intra-uterine life, the child, beginning its existence as a single
cell one two hundred and fiftieth of an inch in diameter, attains in
nine months a weight of seven pounds. The only similar rapidity of
cell formation is seen in certain tumors; although the body may add a
greater amount of weight and in a shorter time, by deposit of fat,
this in but slight measure represents a new formation of tissue, but
is merely a storage of food material in cells. The remarkable repair
and even the new formation of entire parts of the body in the tadpole
will not take place in the completely developed frog.
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