372 CHEMISTR Y OF THE DIGESTIVE PROCESSES. 
a glycogen-like substance was present in the other mucins, which, on 
boiling with a mineral acid, formed a reducing sugar ; this substance he 
supposed to be absent in bile mucin, and hence no reducing sugar was 
formed on heating it with a dilute mineral acid. 
Paijkull 1 afterwards proved that the mucin-like substance which 
gives bile its viscidity really belongs to the nucleo-proteids. If bile be 
precipitated with dilute acetic acid, the presence of the bile salts 
prevents the precipitate from redissolving in excess, and so causes it to 
simulate a mucin ; but if the bile salts are removed by dialysing, or by 
precipitating the substance with alcohol, centrifugalising and quickly 
redissolving in water, the precipitate readily redissolves in excess of 
acetic acid, and in this respect resembles a nucleo-proteid and not a 
mucin. Also, when the substance is precipitated by, and just redissolved 
in, dilute hydrochloric acid, and then subjected to peptic digestion, a 
substance is precipitated, which by its percentage of phosphorus can be 
recognised as similar to the nuclein yielded under Like conditions by 
nucleo-proteids. These facts, together with the much higher percentage 
of nitrogen (14 to 16 per cent.) than mucin which it contains, and its 
failing to yield a reducing sugar on boiling with dilute mineral acids, 
show the substance to be a nucleo-proteid and not a mucin. The quantity 
of this substance present in bile is very variable but always small, 
amounting in ox bile to about one per thousand. 2 
The bile salts. — There are found in bile the salts of a number of 
organic acids of complicated structure, which are closely allied to one 
another ; these salts are collectively called the bile salts. They are not 
found in health in appreciable quantity elsewhere than in the bile, and 
usually occur as sodium salts, except in the bile of some sea fishes, in 
which they are present as potassium salts. 
Since bile is so easily obtainable in quantity, it is not surprising that 
it should early have attracted the notice of the physiological chemist. 
Thenard, in 1809, working with ox bile, was the first to obtain any 
scientific knowledge of the bile acids. He distinguished two com- 
ponents in bile, one precipitated by acetate of lead, which he called 
bile resin, and a soluble part, which he named jncromel. He seems 
to have roughly separated in an impure condition those two most 
commonly occurring bile acids, which we know to-day as glycocholic and 
taurocholic acid, by a method not widely differing from that most used 
at the present time. He precipitated bile with neutral and basic acetates 
of lead, and then extracted the precipitate with nitric acid ; the insoluble 
part left behind, his resin of bile, was impure glycocholic acid. The filtrate 
he reprecipitated with excess of acetate of lead, collected the precipitate, 
and decomposed it by a current of sulphuretted hydrogen, thus obtaining 
his picromcl, which must have corresponded to impure taurocholic acid. 
In 1826, (Imelin published a memoir 3 in which is described a large 
number of bile constituents ; amongst them, one corresponding to 
glycocholic acid, which he obtained in a crystalline form ; and another 
substance, taurine, of which he was the discoverer, although he wrongly 
supposed that it existed ready formed in the bile. The next important 
advance was made by Demarcay, 4 who obtained a substance (Choleinsdure) 
which yielded, on heating with acids, taurine and a resinous substance, 
1 Ztschr.f. physiol. Gh m., Strassburg, 18S8, Bd. xii. S. 196. 9 Paijkull, loc. cit. 
3 " Die Verdauung nach Versuchen." 
4 Ann. d. Chem., Leipzig, 1838, Bd. xxvii. S. 270. 
