170 THE DIGESTIVE FLUIDS. 



amounts from collagen, elastin, and especially silk fibroin. From 

 its occurrence in collagen and its sweet taste it has been termed 

 collagen-sugar (Leimzucker) or glycin (glucin). It is one of the 

 most important decomposition-products which are formed in the 

 nitrogenous metabolism of the animal body, and in part probably 

 gives rise to the formation of urea in mammals and to uric acid in 

 birds and reptiles. Another portion, as we have seen, is eliminated 

 in the bile in combination with cholalic acid ; while a third portion 

 appears in the urine in combination with benzoic acid and phenyl- 

 acetic acid, as hippuric acid and phenaceturic acid, respectively (which 

 see). 



As we have seen, glycocoll is amino-acetic acid. The pure sub- 

 stance crystallizes in the form of colorless rhombohedra or of four- 

 sided prisms, which are readily soluble in water, with difficulty so in 

 warm alcohol, and insoluble in absolute alcohol and ether. It com- 

 bines with acids and alkalies to form salts. The most important of 

 these are the hydrochlorate, which is soluble in water and alcohol, 

 and the copper salt, which results when a boiling solution of glycocoll 

 is added to freshly precipitated cupric hydrate ; the hydrate is thus 

 dissolved, and after concentrating the solution blue needles of the 

 copper salt separate out on cooling. 



Isolation. For purposes of study glycoeoll is best obtained from 

 collagen, on hydrolysis with dilute mineral acids, as will be de- 

 scribed in a subsequent chapter. 



The Bile-pigments. 



The bile-pigments which have been obtained from the bile itself 

 or from biliary concretions are bilirubin, biliverdin, biliprasin, 

 bilifuscin, and others which are less well known. 



Perfectly fresh hepatic bile, in contradistinction to that which is 

 found in the gall-bladder, contains only one pigment, bilirubin, from 

 which all other forms are derived. Such bile is of a golden-yellow 

 color, while bladder-bile usually presents an olive-brown color, owing 

 to the simultaneous presence of its nearest oxidation-product, bili- 

 verdin. A grass-green color is observed when the latter predomi- 

 nates or is exclusively present. 



Bilirubin. Bilirubin results from the decomposition of ha?matin, 

 and normally constitutes a specific product of the activity of the 

 hepatic cells. It appears, however, that the power of transforming 

 the blood-pigment into bilirubin is common to other tissues as well, 

 if we regard the ha3matoidin of Virchow, which is so often found 

 in old extravasations of blood, as identical with bilirubin. Under 

 normal conditions, however, the liver is apparently the only organ 

 of the body in which the formation of bilirubin takes place. 

 Whether or not the final dissolution of disintegrating red corpuscles 

 also occurs at this place has not been decided, but the liver is 

 manifestly capable of retaining the hemoglobin which is thus set 



