Very little can be said on this subject, as we know little about it. The most important urinary pigments, urobilin and hematoporphyrin, and probably the majority of the other normal urinary pigments, are derivatives of the hemoglobin that undergoes metabolic disassimilation. The organism possesses the power of building up hemoglobin from simple molecules (how is unknown), even if no hemoglobin is introduced with the diet. We are unable to influence or to regulate the metabolism of hemoglobin by dietary restrictions; the only thing that we can do is to omit articles of diet containing hemoglobin, for we know that a certain portion of the hemoglobin ingested is absorbed and increases the excretion of bile pigments and the formation of urobilin in the intestine. The latter is absorbed, enters the blood current and is in this way carried to the kidneys.

Hematoporphyrin can hardly be considered a normal metabolic product and the quantities that are eliminated by the kidneys are so small that this substance need not be considered in the treatment of renal disease. I need hardly mention that everything should be avoided that may lead to hematoporphyrinuria; among other things the administration of sulphonal and trional.