ii EXTERNAL DIGESTIVE SECRETIONS 145 



Normally the bile also contains urea, particularly that of the 

 cartilaginous fishes. As regards mineral substances, sulphates are 

 almost entirely absent from the bile, which shows traces of copper, 

 zinc, and particularly of iron, in an amount that varies with the 

 nature of the food. According to Novi, the quantity of iron is 

 less in dogs fed with bread, greatest in a flesh diet ; according to 

 Dastre, the iron content of the bile varies even with a uniform 

 diet, according as the haematopoietic or the haematolytic processes 

 predominate. The iron introduced in a medicinal form is also, 

 according to some authors, retained by the liver and eliminated in 

 the bile (Novi, Kunkel), this being disputed by others (Hamburger, 

 Gottlieb, and Anselm). 



According to Craciunu (1901), the composition of bile varies 

 with age. The bile of young animals up to three years old 

 contains less water and more solids than that of adults (9'8-10'5 

 per cent against 8-8'l per cent solids). In young animals there 

 are also more mucin, more mineral salts, cholesterol, and bile salts ; 

 in adult animals more fats and lecithin. 



Pettenlcofer's Reaction is used to detect the presence of bile acids. The 

 acids or fluids containing them are treated with a little 25 per cent solution 

 of cane sugar, and sulphuric acid is carefully added, so that it forms a layer 

 under the solution. A reddish-purple colour appears at the junction of the 

 liquids, and also where it comes into contact with any froth at the surface. 

 The presence of nitrates may disturb the reaction. 



In testing for bile acids in the blood and urine, the following is the best 

 method : 



Dilute the blood with two volumes of water, and coagulate -the proteins by 

 heating with a few drops of acetic acid. Filter off the coagulum and 

 evaporate the solution on a water-bath. Extract the residue with absolute 

 alcohol which dissolves the bile salts, while the proteins remain undissolved, 

 and evaporate off the alcohol. Dissolve the residue in water containing a 

 little sugar, and add sulphuric acid diluted with an equal volume of water 

 and cooled. On gently warming, the originally cloudy solution clears up, and 

 turns successively orange, yellow, red, and purple. To detect bile acids in 

 urine, it is only necessary to evaporate it to dryness, then extract with 

 alcohol, and proceed as described for blood. 



A more convenient method has recently been introduced, based on the 

 fact that the presence of bile acids enormously increases the surface tension 

 of urine. We shall give this method in detail when discussing the katabolic 

 products of urine. 



Omelin's test for bile pigments. Pour 5 c.c. nitric, with a drop of nitrous 

 acid, into a watch-glass, then carefully introduce the fluid to be examined by 

 a pipette, without allowing it to mix with the reagent. At the point of 

 contact of the two fluids, rings of different colours are formed, which are 

 green, blue, violet, red, and yellow, as they spread from the centre to the 

 periphery. Each colour represents a successive stage in the oxidation of 

 the bile pigment. 



The tests for cholesterol are also interesting. Cholesterol is the chief 

 constituent of the calculi formed in the bile-ducts and gall-bladder, which 

 give rise to hepatic colic. 



1. On adding a few drops of sulphuric acid diluted with one-fifth its 

 volume of water to some cholesterol in a porcelain capsule, a carmine-red 

 colour results (Moleschott). 



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