118 THE BODY AT WORK 



specific gravity ; but during its stay in the gall-bladder it is 

 concentrated by absorption of water, and mucin is added to it. 

 It contains " bile-salts " of complex constitution. These salts 

 favour the solution of certain by-products of cell-metabolism, 

 cholesterin and lecithin ; substances which are formed in 

 many cells, both in animals and plants. Cholesterin occurs 

 most abundantly in nerve-tissue and in blood-corpuscles. 

 Lecithin also is a by-product of the metabolism of nerve- tissue. 

 Protoplasm appears to be incapable of oxidizing these sub- 

 stances, as it does other products of metabolism. Other sub- 

 stances of equally complex constitution are reduced to urea if 

 they contain nitrogen ; to water and carbonic acid if nitrogen 

 be absent. Cholesterin and lecithin have to be eliminated 

 without further change. Some of the cholesterin is excreted 

 by the sebaceous glands of the skin. It is the chief constituent 

 of "lanoline" prepared from sheep's wool; an unguent which 

 owes its valuable properties to the resistance which cholesterin 

 offers to cell action, and therefore to the action of living fer- 

 ments. Bacteria cannot turn it rancid. The sebaceous 

 glands have the power of directing metabolism into a channel 

 in which cholesterin is the chief product, but apparently all 

 cells make it in small quantity. The bile-salts carry choles- 

 terin and lecithin into the alimentary cajial, from which they 

 are not reabsorbed. Some of the bile-salts are lost to the body, 

 but the remainder re-enter the circulation, and recommence 

 their work as vehicles for these inoxidizable and insoluble 

 substances. In the gall-bladder cholesterin is apt to separate 

 out from the bile in the form of gall-stones ; but whether this is 

 due to an excess of cholesterin in the bile, or to an abnormal, 

 inflammatory condition of the lining membrane of the gall- 

 bladder, is still an open question. 



Bile also contains bile-pigments. Their colour varies in 

 different animals, and changes according as the bile is exposed 

 to the air, or subject to the action of reducing agents. If 

 oxidized, the colour is green (biliverdin) ; if reduced, brownish- 

 yellow (bilirubin). Bile-pigment is formed from haemoglobin, 

 the colouring matter of the blood, after the removal of its iron. 

 Worn-out red blood-corpuscles are destroyed in the spleen, 

 in the manner already described, but it is uncertain whether 

 the conversion of the haemoglobin thus set free into bilirubin 



