from disease; changes in the
tissues of the same nature, or closely akin to those which are found
in disease, are constantly occurring in a state of health. The
importance of parasites in causing disease has led to the conception
of disease as almost synonymous with parasitism; but it must be
remembered that the presence of parasites living at the expense of the
body is perfectly consistent with a state of health. Degeneration,
decay and parasitism only become disease factors when the conditions
produced by them interfere with the life which is the normal or usual
for the individual concerned.

All the changes which take place in the cells are of great importance
in conditions of both health and disease, for life consists in
cooerdinated 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 haemoglobin which