CHEMISTRY OF DIGESTION AND NUTRITION. 325 



occurs in variable quantities. Cholesterin is very widely distributed in the 

 body, being found especially in the white matter (medullary substance) of 

 nerve-fibres. It seems, moreover, to be a constant constituent of all animal 

 and plant cells. It is assumed that cholesterin is not formed in the liver, 

 but that it is eliminated by the liver-cells from the blood, which collects it 

 from the various tissues of the body. That it is an excretion is indicated by 

 the fact that it is eliminated unchanged in the feces. Cholesterin is insoluble 

 in water or in dilute saline liquids, and is held in solution in the bile by 

 means of the bile-acids. We must regard it as a waste product of cell-life, 

 formed probably in minute quantities, and excreted mainly through the 

 liver. It is partly eliminated through the skin, in the sebaceous and sweat 

 secretions, and in the milk. 



Lecithin, Fats, and. Nucleo-albumin. — Lecithin also seems to be present, 

 generally in small quantities, in the cells of the various tissues, but it occurs 

 especially in the white matter of nerve-fibres. It is probable, therefore, that, 

 so far as it is found in the bile, it represents a waste product formed in 

 different parts of the body and eliminated through the bile. The special 

 importance, if any, of the small proportion of fats and fatty acids in the bile 

 is unknown. The ropy, mucilaginous character of bile is due to the presence 

 of a body formed in the bile-ducts and gall-bladder. This substance was 

 formerly designated as mucin, but it is now known that in ox-bile at least 

 it is not a true mucin, but is a nucleo-albumin (see Chemical section). Ham- 

 marsten reports that in human bile some true mucin is found. Outside the 

 fact that it makes the bile viscous, this constituent is not known to possess 

 any especial physiological significance. 



General Physiological Importance of Bile. — The physiological value 

 of bile has been referred to in speaking of its several constituents, but it will 

 be convenient here to restate these facts and to add a few remarks of general 

 interest. Bile is of importance as an excretion in that it removes from the 

 body waste products of metabolism, such as cholesterin, lecithin, and bile- 

 pigments. With reference to the pigments, there is evidence to show that a 

 part at least may be reabsorbed while passing through the intestine, and be 

 used again in some way in the body. The bile-acids represent end-products 

 of metabolism involving the proteids of the liver-cells, but they are undoubt- 

 edly reabsorbed in part, and cannot be regarded merely as excreta. As a 

 digestive secretion the most important function attributed to the bile is the 

 part it takes in the digestion of fits. Tn the first place, it aids in the splitting 

 of a part of the neutral fats and the subsequent emulsification of the re- 

 mainder (p. 307). More than this, bile aids materially in the absorption of the 

 digested fats. A number of observers have shown that when a permanent 

 biliary fistula is made, and the bile is thus prevented from reaching the intes- 

 tinal canal, a large proportion of the flit of the food escapes absorption and 

 is found in the \\'t-vs. This property of the bile IS known to depend upon 

 the bile-acids it contains, but how they act is not clearly understood. It was 

 formerly believed, on the basis of some experiments by von Westinghausen, 



