_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 hæmorrhage 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