It will be clear that to obtain the antitoxine we must depend upon some natural method for its production.  We do not know enough of the chemical nature of the antitoxines to manufacture them artificially.  Of course we can not deny the possibility of their artificial production, and certain very recent experiments indicate that perhaps they may be made by the agency of electricity.  At present, however, we must use natural methods, and the one commonly adopted is simple.  Some animal is selected whose blood is harmless to man and that is subject to the disease to be treated.  For diphtheria a horse is chosen.  This animal is inoculated with small quantities of the diphtheria poison without the diphtheria bacillus.  This poison is easily obtained by causing the diphtheria bacillus to grow in common media in the laboratory for a while, and the toxines develop in quantity; then, by proper filtration, the bacteria themselves can be removed, leaving a pure solution of the toxic poison.  Small quantities of this poison are inoculated into the horse at successive intervals.  The effect on the horse is the same as if the animal had the disease.  Its cells react and produce a considerable quantity of the antitoxine which remains in solution in the blood of the animal.  This is not theory, but demonstrated fact.  The blood of a horse so treated is found to have the effect of neutralizing the diphtheria poison, although the blood of the horse before such treatment has no such effect.  Thus there is developed in the horse’s blood a quantity of the antitoxine, and now it may be used by physicians where needed.  If some of this horse’s blood, properly treated, be inoculated into the body of a person who is suffering from diphtheria, its effect, provided the theory of antitoxines is true, will be to counteract in part, at least, the poisons which are being produced in the patient by the diphtheria bacillus.  This does not cure the disease nor in itself drive off the bacilli, but it does protect the body from the poisons to such an extent as to enable it more readily to assert its own resisting powers.

This method of using antitoxines as a help in curing disease is very recent, and we can not even guess what may come of it.  It has apparently been successfully applied in diphtheria.  It has also been used in tetanus with slight success.  The same principle has been used in obtaining an antidote for the poison of snake bites, since it has appeared that in this kind of poisoning the body will develop an antidote to the poison if it gets a chance.  Horses have been treated in the same way as with the diphtheria poison, and in the same way they develop a substance which neutralizes the snake poison.  Other diseases are being studied to-day with the hope of similar results.  How much further the principle will go we can not say, nor can we be very confident that the same principle will apply very widely.  The parasitic diseases are so different in nature that