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 hæmoglobin 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