DIGESTION AND THE DIGESTIVE AGENTS. 345 



such a test tube the colors will arrange themselves one 

 above the other in the order indicated, the most advanced 

 stage of oxidation being of course next to the nitric acid, 

 the least advanced stage of oxidation on top of the liquid. 

 This characteristic re-action for the detection of bile pig- 

 ments, familiar to all physiological laboratories, is called 

 "Gmelin's re-action. " It is a very sensitive one, and by 

 means of it the presence of bile pigments has been detected 

 in other liquids of the body, as for instance, the secretion 

 from the kidneys. The bile pigments are of especial inter- 

 est, because it is fairly well established that they are derived 

 from haemoglobin. When the red corpuscles break down 

 in the circulation or disintegrate in the spleen or liver, the 

 colored haemoglobin is sent to the liver, and by the liver 

 cells is converted into bilirubin or biliverdin. Haemoglobin 

 contains iron, but bilirubin and biliverdin are iron -free. 

 This shows that the iron in the haemoglobin must be re- 

 tained in the liver, and possibly dropped back into the blood 

 and sent to the marrow of the bones, to be used anew in 

 the formation of fresh haemoglobin. The amount, there- 

 fore, of bilirubin or biliverdin eliminated in the bile would 

 give us a clue to the rapidity of corpuscular disintegra- 

 tion. 



Bile is practically a watery solution of certain mineral 

 salts which holds in solution goodly quantities of the or- 

 ganic bile salts. These organic bile salts hold in solution 

 the cholesterin of the bile, and finally the bile pigments to 

 which the color is due. In addition to these main constitu- 

 ents there are found in bile traces of fats and soaps, and a 

 peculiar substance called lecithin, which is interesting be- 

 cause it is always found in nervous tissues and characterized 

 by containing phosphorus. It is no doubt a mere disinte- 

 gration product resulting from the activity of these tissues 

 and sent to the bile merely to be removed as a waste 

 product. 



The reason for the presence of the fats and soaps in the 

 bile is still missing. 



