CH. XXXV.] THE BILE CIRCULATION 535 



at once raised. It is on these experiments that the theory of a bile 

 circulation is mainly founded. The bile circulation relates, however, 

 chiefly, if not entirely, to the bile salts : they are found but sparingly 

 in the faeces ; they are only represented to a slight extent in the urine : 

 hence it is calculated that seven-eighths of them are reabsorbed from 

 the intestine. Small quantities of cholalic acid, taurine, and glycine 

 are found in the faeces; the greater part of these products of the 

 decomposition of the bile salts is taken by the portal vein to the 

 liver, where they are once more synthesised into the bile salts. Some 

 of the taurine is absorbed and excreted as tauro-carbamic acid in the 

 urine. Some of the absorbed glycine may be excreted as urea. The 

 pigment is changed into stercobilin and leaves the body partly in 

 the faeces, but some is absorbed and is finally excreted as urobilin 

 in the urine. The cholesterin in the faeces was formerly supposed 

 to be a bile-residue; but in some animals, especially those which 

 feed on grass, the source of the faecal cholesterin is the phytosterin 

 (vegetable cholesterin) in the food. In some cases it is reduced and 

 forms a derivative termed coprosterin (Austin Flint's stercorin). 



The bile-expelling mechanism must be carefully distinguished 

 from the bile-secreting action of the liver-cells. The bile is forced 

 into the ducts, and ultimately into the duodenum, by the pressure of 

 newly-formed bile pressing on that previously in the ducts, and this 

 is assisted by the contraction of the plain muscular fibres of the 

 larger ducts and gall-bladder, which occurs when the food enters the 

 duodenum. In cases of obstruction, as by a gall-stone, in the ducts, 

 this action becomes excessive, and gives rise to the intense pain 

 known as hepatic colic. 



Many cholagogues (bile-drivers), such as calomel, act on the bile- 

 expelling mechanism and increase the peristalsis of the muscular 

 tissue ; they do not really cause an increased formation of bile. 



Jaundice. The commonest form of jaundice is produced by 

 obstruction in the bile-ducts preventing the bile entering the 

 intestine. A very small amount of obstruction, for instance, a 

 plug of mucus produced in excess owing to inflammatory processes, 

 will often be sufficient, as the bile is secreted at comparatively low 

 pressure. Under these circumstances, the faeces are whitish or clay 

 coloured, and the bile passing backwards into the lymph,* enters 

 the blood and is thus distributed over the body, causing a yellow 

 tint in the skin and mucous membranes, and colouring the urine. 



In some cases of jaundice, however (e.g., produced by various 

 poisons), there is no obvious obstruction; the causes of non- 

 obstructive, or blood-jaundice, form a pathological problem of some 

 interest. Some years ago it was believed that the bile pigment was 



* The absorption is by the lymph, because if jaundice is produced in an 

 animal by ligature of the bile duct, it will cease when the thoracic duct is tied, 



