DIGESTION 197 



works on chemistry. When obtained in the pure state it undergoes crystal- 

 lization when it presents itself in the form of flat rectangular crystals which 

 are soluble in ether and hot alcohol. In the bile it is held in solution by 

 the bile salts. Chemic analysis shows that it is a monatomic unsaturated 

 alcohol with the formula C 2 7H 4 50H. Cholesterol, though a constant con- 

 stituent of bile, is not confined to this fluid as it has been shown to be a 

 normal constituent of all animal and vegetable cells, though it is particularly 

 abundant in the myelin of nerve-fibers. Though cholesterol has for a long 

 time been regarded merely as one of the products of the katabolism of living 

 material, it has come to be believed that it is necessary to the vitality of tissue 

 cells and especially to the blood cells. Entering into the composition of 

 the surface layer of cells, it prevents the entrance of certain toxins which 

 would have a destructive influence on their structure or composition. In 

 the metabolism of cells it is set free after which it passes into the blood to be 

 secreted by the liver. In the bile it frequently undergoes crystallization and 

 forms one of the forms of gall-stones. In the intestine it is converted into 

 stercorin and discharged in the feces. 



Bilirubin, Biliverdin. These two pigments impart to the bile its 

 red and green colors respectively. Bilirubin is present in the bile of human 

 beings and the carnivora, biliverdin in the bile of the herbivora. As the 

 former pigment readily undergoes oxidation in the gall-bladder, giving rise 

 to the latter pigment, almost any specimen of bile may present any shade 

 of color between red and green. Bilirubin is regarded as a derivative of 

 hematin, one of the cleavage products of hemoglobin, the coloring-matter 

 of the blood. In the liver the hematin combines with water, loses its iron, 

 and is changed to bilirubin. By continuous oxidation there are formed 

 biliverdin, bilicyanin, and choletelin. After their discharge into the intes- 

 tine the bile pigments are finally reduced to hydrobilirubin or an allied sub- 

 stance, stercobilin, which becomes one of the constituents of the feces. A 

 portion of the latter is absorbed into the blood and ultimately discharged 

 into the urine where it is known as urobilin. The two substances are re- 

 garded as identical. An oxidation of the bilirubin can be produced by 

 nitroso-nitric acid. If this agent is added to a thin layer of bile on a porce- 

 lain surface, a series of colors will rapidly succeed one another, commencing 

 with green and passing to blue, orange, purple, and yellow. This is the 

 basis of the well-known test for bile pigments suggested by Gmelin. 



Lecithin. Lecithin is regarded, because of its physical properties and 

 chemic composition, as a complex fat. When pure it presents itself gener- 

 ally as a white crystalline powder, though very frequently as a white waxy 

 mass which is soluble in ether and alcohol. Its chemic formula is C4 4 H 90 - 

 NPO 9 . Lecithin is widely distributed throughout the body, being found in 

 blood, lymph, red and white corpuscles, nerve-tissue, yolk of egg, semen, 

 milk, and bile. Lecithin has been regarded as one of the decomposition 

 products of nerve-tissue, removed from the blood by the liver and thus be- 

 coming one of the constituents of the bile, in which it is held in solution by 

 the bile salts. Lecithin can be readily decomposed by various agents yield- 

 ing glycophosphoric acid, a fat acid and cholin. 



The Mode of Secretion and Discharge of Bile. The manner in which 

 the bile flows from the liver into the main hepatic ducts, the variations in the 

 rate of its discharge into the intestine, as well as the total quantity secreted 



