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Page 11
All the changes which take place in the cells are of great importance
in conditions of both health and disease, for life consists in
co�rdinated cell activity. The activities of the cells can be divided
into those which are nutritive, those which are functional and those
which are formative. In the functional activity the cell gives off
energy, this loss being made good by the receipt of new energy in the
form of nutritive material with which the cell renews itself. In
certain cells an exact balance seems to be maintained, but in those
cells whose activity is periodic function takes place at the expense
of the cell substance, the loss being restored by nutrition during the
period of repose. This is shown particularly well in the case of the
nerve cells (Fig. 13). Both the functional and nutritive activity can
be greatly stimulated, but they must balance; otherwise the condition
is that of disease.
[Illustration: FIG 13.--NERVE CELLS OF AN ENGLISH SPARROW (_a_) Cells
after a day's full activity, (_b_) cells after a night's repose. In
(_a_) the cells and nuclei are shrunken and the smaller clear spaces
in the cells are smaller and less evident than in (_b_). (Hodge)]
The formative activity of cells is also essential to the normal state.
Destruction of cells is constantly taking place in the body, and more
rapidly in certain tissues than in others. Dried and dead cells are
constantly and in great numbers thrown off from the surface of the
skin: such epidermic appendages as the hair and nails grow and are
removed, millions of cells are represented in the beard which is daily
removed. Cells are constantly being destroyed on the intestinal
surface and in the glands. There is an enormous destruction of the
blood cells constantly taking place, certain essential pigments, as
that of the bile, being formed from the h�moglobin which the red blood
corpuscles contain and which becomes available on their destruction.
All such loss of cells must be made good by the formation of new ones
and, as in the case of the nutritive and functional activity, the loss
and renewal must balance. The formative activity of cells is of great
importance, for it is by means of this that wounds heal and diseases
are recovered from. This constant destruction and renewal of the body
is well known, and it is no doubt this which has given rise to the
belief, widely held, that the body renews itself in seven years and
that the changes impressed upon it by vaccination endure for this
period only. The truth is that the destruction and renewal of most
tissues in the body takes place in a much shorter interval, and, as we
shall see, this has nothing to do with the changes concerned in
vaccination. All these activities of the cells vary in different
individuals, in different parts and at different ages.
The lesions or injuries of the body which form so prominent a part of
disease vary in kind, degree and situation, depending upon the
character of the injurious agent, the duration of its action and the
character of the tissue affected. The most obvious injuries are those
produced by violence. By a cut, blood vessels are severed, the
relations of tissues disturbed, and at the gaping edges of the wound
the tissue usually protected by the skin is exposed to the air,
resulting in destruction of the cells contained in a thin layer of the
surface. The discoloration and swelling of the skin following a blow
is due to rupture of vessels and escape of blood and fluid, and
further injury may result from the interruption of the circulation.
By the application of heat the tissue may be charred and the albumen
of the blood and tissue fluids coagulated. Living cells are very
susceptible to the action of heat, a temperature of 130 degrees being
the thermal death point, and even lower temperatures are fatal when
their action is prolonged. The action of the heat may produce definite
coagulation of the fluid within the cells in the same way that the
white of an egg is coagulated. Certain of the albumens of the body
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
h�moglobin of the red corpuscles so that the function of these as
oxygen carriers is stopped.
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