The entire amount of iron in the blood of an adult person is, on the average under normal conditions, four grams, as much as a nickel weighs. We may well judge that this amount is not sufficient to set the motive power of our bodies in action, if we overlook that complex factor the circulation of blood. The left side of the heart has the capacity of containing about six ounces of blood, and every heart beat drives this amount through the aorta. With seventy beats to the minute, twenty-five pounds of blood is pumped from the heart every minute. What is the result? That the four grams of iron keep up such an incessant movement that they pass from the heart into the aorta sixty times an hour or 1440 times in 24 hours. It may be asserted, therefore, that in 24 hours 13 pounds of iron (that is 1440x4 grams) pass from the heart into the aorta. Can it be doubted, in view of this, that the iron serves to produce an electro-dynamic force? In respect to the generation of electricity, it matters not whether there be an entirely new supply of iron passing a given point, or whether the same iron pass that point anew each minute. Two factors work together in the circulation of the blood, namely, the active attraction of nerve tissue and the passive susceptibility of the blood contents to that attraction. Faraday has conclusively shown that blood is magnetic in character because of the iron it contains. If four grams of iron is the normal quantity in the blood, it is clear that the reduction of this amount, say by two grams, will lessen its susceptibility and slacken its circulation. The electrical nerve ends will then strain in vain for the electricity which the blood current should yield, and the result will be neuralgia. It is the magnetic iron in haemoglobin which makes every sort of nervous function possible, in the cerebral (brain) and in the sympathetic (intestinal) tracts, and since it is thus made clear that intellectual activity on the one hand and breathing and digestion and excretion on the other are dependent on the iron content of the blood, we must also recognize that, as iron attends every nerve action, the secretion of urine too takes place under the influence of haemoglobin. Insofar as haemoglobin hastens the departure of the excrementitious matter in urine out of the system, there is a daily loss of iron in the urine. This loss in the form of urohaematin may total four centigrams, or a hundredth part of our supply.