FOOD AND DIGESTION. 397 



creted to a slight extent in the urine as tanro-carbamic acid, but it is 

 probable that although part of this may unite to re-form glycocholic or 

 taurocholic acid, the remainder is united with oxygen, and is burnt off 

 in the form of carbonic acid and water. 



A substance, contained in the faeces, and named stercorin, is closely 

 allied to cholesterin. Ten grains and a half of stercorin are excreted 

 daily (A. Flint). 



From the peculiar manner in which the liver is supplied with much 

 of the blood that flows through it, it is probable that this organ is ex- 

 cretory, not only for such hydro-carbonaceous matters as may need ex- 

 pulsion from the blood, but that it serves for the direct purification of 

 the stream which, arriving by the portal vein, has just gathered up vari- 

 ous substances in its course through the digestive organs substances 

 which may need to be expelled almost immediately after their absorp- 

 tion. For it is easily conceivable that many things may be taken up 

 during digestion, which not only are unfit for purposes of nutrition, but 

 which would be positively injurious if allowed to mingle with the gen- 

 eral mass of the blood. The liver, therefore, may be supposed placed in 

 the only road by which such matters can pass unchanged into the general 

 current, jealously to guard against their further progress, and turn them 

 back again into an excretory channel. The frequency with which me- 

 tallic poisons are either excreted by the liver, or intercepted and retained, 

 often for a considerable time, in its own substance, may be adduced as 

 evidence for the probable truth of this supposition. 



The secretion of the bile by the hepatic cells is undoubtedly influenced 

 by the amount of blood supplied to them. This is well seen after a meal, 

 when the amount of blood passing through the portal circulation in con- 

 sequence of the congestion of the secreting organs of the abdomen is 

 greatly increased, and with it the bile secretion. It is, however, probable 

 that the secretion of the cells is in some more direct way under the con- 

 trol of the nervous system, but how this influence is exercised is un- 

 known. The antecedents of the various substances of the bile from 

 which the cells manufacture its chief constituents are not exactly known. 

 It is surmised that the bilirubin is formed from hemoglobin brought 

 from the spleen either actually dissolved in the plasma of the blood or 

 in such a condition in the corpuscles as to be easily acted upon by the 

 liver cells, by which the iron is separated. The bile salts are, at any 

 rate in part, formed simply by the conjunction of glycinand taurin with 

 cholalic acid, all of which may be brought to the liver in the portal 

 blood, but failing this it is probable that the hepatic cells can produce 

 these substances anew. 



