The principal ingredient in the formation of uric acid is nitrogen, one of the six elements which enter into all proteid or albuminous food materials, also called nitrogenous foods. Uric acid, as one of the by-products of digestion, is therefore always present in the blood and, in moderate quantities, serves useful purposes in the economy of the human and animal organism like the other waste materials. It becomes a source of irritation and cause of disease only when it is present in the circulation or in the tissues in excessive amounts. How Uric Acid Is Precipitated The alkaline blood takes up the uric acid, dissolves it and holds it in solution in the circulation until it is carried to the organs of depuration and eliminated in perspiration and urine. If, however, through the excessive use of nitrogenous foods or defective elimination, the amount of uric acid in the system is increased beyond a certain limit, the blood loses its power to dissolve it and it forms a sticky, glue-like, colloid substance, which occludes or blocks up the minute blood vessels (capillaries), so that the blood cannot pass readily from the arterial system into the venous circulation. This interference with the free passing of the blood is greater in proportion to the distance from the heart, because the farther from the heart, the less the force behind the circulation. Therefore we find that slowing up of the blood currents, whether due to uric acid occlusion or any other cause, is more pronounced in the surface of the body and in the extremities than in the interior parts and organs. This occlusion of the surface circulation can be easily observed and even measured by a simple test. Press the tip of the forefinger of one hand on the back of the other. A white spot will be formed where the blood has receded from the surface on account of the pressure. Now observe how quickly or how slowly the blood returns into this white patch. Dr. Haig says that, if the reflux of the blood take place within two or three seconds, the circulation is normal and not obstructed by uric acid. If, however, the blood does not return for four or more seconds, it is a sign that the capillary circulation is obstructed by colloid uric acid occlusion. In this connection I would call attention to the fact that the accumulation of carbonic acid in the cells and tissues, and the resulting oxygen starvation, may produce similar interference with the circulation and result in the same symptoms, including the slow reflux of blood after pressure, as those which Dr. Haig ascribes to the action of uric acid only.