THE DIGESTIVE SYSTEM. 321 



in the liver. The larger proportion of the bile salts which pass into 

 the intestine is reabsorbed and returns to the liver to enter again into 

 the composition of bile. This is spoken of as the circulation of the 

 bile salts, and not only do these substances enter once more into the 

 formation of bile, but they also stimulate the liver to further secretion, 

 that is, they act as cholagogues. A small proportion of the bile salts 

 is excreted in the faeces in the form of dyslysin, a substance formed 

 from them by bacterial decomposition. Part of the sulphur excretion 

 from the body takes place in this way, since this element forms part of 

 the taurine molecule. 



Cholesterol (p. 8) and lecithin (p. 8) are probably largely derived 

 from the stroma of the red blood corpuscles which are broken down 

 in the liver. Gall-stones, which are of fairly common occurrence in 

 the gall-bladder, are usually composed chiefly of cholesterol. Normally 

 the latter substance is excreted in the faeces. 



The bile pigments are characterised by their colour and by the fact 

 that they are easily oxidised by nitrous acid, yielding a series of 

 coloured products (Grnelin's test). If nitric acid containing nitrous 

 acid is added to bile in which the pigment present is bilirubin, the 

 colour changes to green (biliverdin), then to blue, violet, red, and 

 finally to yellow (choletelin). Bilirubin and biliverdin show no 

 absorption bands with the spectroscope. Bilirubin (C 16 H 18 N 2 3 ) 

 is identical with hsematoidin, a substance formed by the decomposition 

 of haemoglobin in old blood clots in the body. Its molecule only 

 differs from that -of iron-free haematin, formed by the action of strong 

 mineral acids on haemoglobin, in possessing one atom less of oxygen. 



Bile pigments are derived from the pigment of the blood by the 

 breaking down of the latter in the liver. The facts upon which this 

 statement is based are (1) the identity of bilirubin with haematoidin, 

 (2) histological observations on the liver, (3) observations on the 

 proportion which exists between the rate of destruction of the red 

 blood corpuscles and the amount of bile pigment formed, and (4) the 

 effect of injection of haemoglobin into the blood stream. Histological 

 observations show that the walls of the sinusoids in the liver are 

 incomplete, so that red blood corpuscles can pass through them ; and 

 red corpuscles, broken up to a greater or less degree, have been seen 

 within the liver cells. Moreover, the presence of iron in the liver 

 cells can be demonstrated by the blue colour produced by treatment 

 of sections with ferrocyanide of .potassium and hydrochloric acid. 

 Again, when the destruction of red blood corpuscles is increased, as 

 in the disease known as pernicious anaemia, or in poisoning by the 

 injection of pyrogallic acid or toluylene diamine, the amount of bile 



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