DIGESTION 337 



Both acids are made up of a non-nitrogenous body, cholic or 

 cholalic acid, and a nitrogenous body, glycin or glycocoll in glyco- 

 cholic, and taurin in taurocholic acid. 



The decomposition of the bile-acids into these substances is 

 effected by boiling them with dilute acid or alkali, a molecule of 

 water being taken up ; thus 



CaflHuNOp + H 2 =C 2 H 5 NO 2 + C^H^O, ; 



Glycocholic acid. Glycin. Cholic acid. 



C 2C H 45 NS0 7 + H 2 =C 2 H 7 NS0 3 + C 24 H 40 O 5 . 



Tanrocholic acid. Taurin. Cholic acid. 



Taurocholic acid is much more easily split up than glycocholic ; 

 even boiling with water is sufficient. 



Glycin, as already stated, is amino-acetic acid, taurin is amino- 

 ethyl-sulphonic acid, an atom of hydrogen being in each case 

 replaced by NH 2 . A notable difference between glycocholic and 

 taurocholic acid is that the latter contains sulphur. The whole of 

 this belongs to the taurin. Both glycin and taurin are derived from 

 the disintegration of proteins, and the sulphur of the taurin repre- 

 sents a portion of the sulphur of the proteins. We have already 

 seen that among the products of protein hydrolysis a sulphur- 

 containing body, cystin, is found, and there is good evidence that 

 taurin is derived from cystin. 



Traces of cholic acid, formed by hydrolysis from the bile-acids by 

 the action of putrefactive bacteria, are found in the intestines, 

 especially in the lower portion. 



Pettenkofer' s test for bile-acids (Practical Exercises, p. 430), acci- 

 dentally discovered in examining the action of bile upon sugar, 

 depends upon three facts : (i) That cholic acid and furfuraldehyde 

 give a purple colour when brought together ; (2) that the bile-salts 

 yield cholic acid when acted upon by sulphuric acid ; (3) that when 

 cane-sugar is decomposed by strong sulphuric acid, furfuraldehyde 

 is formed. 



Since a similar colour is given when the same reagents are added 

 to a solution containing albumin, it is necessary to remove this, if 

 present, from any liquid which is to be tested for bile-acids. 



Lecithin and cholesterin are by no means peculiar to bile (p. 4). 

 They are very widely distributed in the body. Lecithin belongs to 

 the group of phosphatides, fat-like phosphorus-containing substances 

 present in all cells. It is a compound of glycerin with two molecules 

 of fatty acid and one of phosphoric acid. The phosphoric acid is 

 at the same time united with a base cholin. The fatty acid (stearic, 

 palmitic, oleic, etc.) varies in different varieties of lecithin. Heated 

 with baryta-water, lecithin yields the corresponding fatty acid in 

 the form of a soap, along with cholin and glycero-phosphoric acid* 



Cholesterin is an alcohol with the empirical formula C.^H^O. It is 

 best obtained from white gall-stones, of which it is the chief, and some- 

 times almost the sole constituent (see Practical Exercises, p. 431). 



The chief inorganic salts of bile are sodium chloride, sodium car- 

 bonate, and alkaline sodium phosphate. The phosphoric acid of the 

 ash comes partly from the phosphorus of organic compounds (leci- 

 thin and bile-mucin), the sulphuric acid from the sulphur of tauro- 

 cholic acid, the sodium largely from the bile-salts. Iron is a notable 

 inorganic constituent of bile, although it exists only in traces, in the 

 form of phosphate of iron. Manganese is also present in minute 

 amount. 100 c.c. of fresh bile yields 50 to 100 c.c. of carbon dioxide, 

 part of which is in solution and part combined with alkalies. 



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