1. Manganese dioxide suspended in an acacia-sodium chloride solution provides a non-toxic injection which in the present experiments has contained no particles larger than 1µ and which, when deposited in the body, can be determined quantitatively and seen microscopically.

2. Intravenous injections have been made under precautions which preclude removal from the blood or deposition in organs through simple capillary blockage.

3. In nine experiments out of thirteen the circulating blood contained no manganese after 18 minutes. In the four remaining instances there was a steady slight elimination which was incomplete at the end of 1 hour. Within certain limits the rate of removal from the circulating blood and the sites of deposition in the animal are not influenced by the concentration of the suspension, the blood pressure, or antecedent introduction of acacia or histamine.

4. In the cat amounts of manganese dioxide varying between 9.8 and 3.9 mg. of manganese and containing from 50,000,000,000 to 10,000,000,000 particles, if injected intravenously, permit recovery at the end of 1 hour of 90 per cent of the material in the lungs, liver, and spleen in the following proportions: lungs 47 per cent; liver 38.3 per cent; spleen 4.3 per cent.

5. These experiments, coupled with correlative results by other investigators, make it clear that in certain organs—the lungs, liver, and spleen of the cat—the vascular endothelium possesses phagocytic power rendering the capillaries permeable to particulate material as well as to gases, liquids, and dissolved substances.

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