DIGESTION. 57 



by a further addition of sulphuric acid, and. the solution assumes 

 a bright cherry color, changing to violet, and, if much bile be 

 present, to deep purple. 



(2) Coloring matter of the bile, biliverdin and bilirubin. Both 

 pigments are found in human bile, but the former is characteristic 

 of the bile of herbivora, and the latter, bilirubin, of the bile of 

 carnivora. The pigments are crystallizable, and are insoluble in 

 water. The crystals have the green and red colors of the pigments. 

 Test, " Gmelin's bile test :" Add fuming nitric (nitroso-nitric) acid, 

 and there results a play of colors which is best seen when the bile 

 solution is in thin layer on a white plate. The presence of the 

 bile-pigments is shown also by absorption-bands in the spectrum. 

 Bilirubin is probably derived from haemoglobin, and biliverdin from 

 the bilirubin, as they are chemically closely allied. 



(3) Cholesterm, a crystallizable, insoluble substance which belongs 

 to the alcohol group in chemical composition. Best recognized by 

 microscopic appearance of crystals (Fig. 10), though it may be 

 tested chemically by the addition of sulphuric acid, which gives a 

 red reaction. 



What are the functions of the liver ? 



This must be considered to be still a somewhat unfinished prob- 

 lem, but we can safely assume three duties : (1) that of excretion, 

 (2) as an element in the process of digestion, (3) the elaboration of 

 absorbed food before passing it into the blood-circulation. Of these 

 uses, (1) and (2) are dependent upon the secretion of bile : (3) is an 

 intrinsic property of the organ. 



What excretory function has the liver? 



The bile for the most part, in normal conditions, is a sort of circu- 

 lating fluid : it is secreted by the liver, poured into the intestines, 

 and reabsorbed from them, to be returned through the portal vein 

 to the liver for recirculation. There is, however, a small propor- 

 tion of biliary matter, about one-sixteenth, which is not absorbed, 

 and this consists chiefly of the pigments of the bile. The salts 

 are nearly all reabsorbed in the assimilation. Further than this, 

 the liver is found to, so to speak, filter materials which would 

 be poisonous if circulating in the general system, and either to 

 reject them at once or to store them up and reject them slowly 

 back to the intestine. The excrementitious material from the liver 

 is known as stercobilin. Stercorin is found in the faeces, and is 

 thought to be an excretion of the liver : it closely resembles cho- 



