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 haemorrhage 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.