518 THE HUMAN BODY 



The urea formed thus from the ammonia compounds of the blood 

 belongs to the group of exogenous excreta, since it does not repre- 

 sent a product of true cell metabolism in the Body. From the 

 liver it is delivered to the blood of the general circulation where it 

 remains till excreted by the kidneys. 



The direct excretory function of the liver consists in the with- 

 drawal from the blood and the delivery to the intestine through 

 the bile of certain endogenous excretory substances. The most 

 marked of these are the bile-pigments, which, as stated in Chap. 

 XVII, are derived from the worn-out red corpuscles of the blood, 

 and consist essentially of the pigment portion of hemoglobin minus 

 its iron. Two bile-pigments occur, of very similar chemical con- 

 stitution; bilirubin, golden-brown in color, is the predominating 

 pigment of carnivorous bile, and of human bile on a mixed diet; 

 biliverdin, a green pigment, predominates in the bile of herbiverous 

 animals. Recent investigations have indicated that the bile- 

 pigments, although primarily waste products, serve some useful 

 purpose during their stay in the alimentary tract. The nature of 

 their use is not yet clear. 



Beside the bile-pigments the liver excretes small amounts of 

 various substances which are interesting chiefly on account of 

 their insolubility in the ordinary fluids of the Body, and the fact 

 that they are soluble in bile. These are found in the Body for the 

 most part in nervous tissues, and they may be excretory products 

 of nerve-cell metabolism. The most abundant of them is the non- 

 nitrogenous substance cholesterin. 



The chief constituents of bile not heretofore mentioned are the 

 bile salts, sodium salts of peculiar acids found only- in bile, glyco- 

 cholic add and taurocholic add. These do not appear to be excreta 

 pure and simple, inasmuch as they are reabsorbed in part by the 

 intestinal walls, and returned by the portal vein to the liver whence 

 they again appear as constituents of the bile. They are thought 

 to give to bile its special ability to promote fat absorption by dis- 

 solving the fatty acids, and it is also by virtue of their presence 

 that the bile is able to dissolve cholesterin. 



General Arrangement of the Urinary Organs. These consist 

 of (1) the kidneys, the glands which secrete the urine; (2) the 

 ureters or ducts of the kidneys, which carry their secretion to 

 (3) the urinary bladder, a reservoir in which it accumulates and 



