464 THE HUMAN BODY 



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

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

 where it floats about till filtered out 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 wornout 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 constitution; 

 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. 



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 acid and taurocholic acid. 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 

 from which it is expelled from time to time through (4) an exit 

 tube, the urethra. The general arrangement of these parts, as 

 seen from behind, is represented in Fig. 138. The two kidneys, 

 R, lie in the dorsal part of the lumbar region of the abdominal 

 cavity, one on each side of the middle line. Each is a solid mass, 

 with a convex outer and a concave inner border, and its upper end 



