CHAP, i.] DIGESTION. 247 



cipitate thus formed, readily curdles milk, but even when acidified 

 has no peptic action on proteids, shewing that the precipitate caused 

 by the addition of the magnesium carbonate has carried down all 

 the pepsin but left behind at least a good deal of the rennet-ferment. 



Rennet-ferment seems to be present in variable quantity in the 

 gastric juice of most animals, and may also be obtained from the 

 gastric mucous membrane of many though not all animals. It is 

 especially abundant in the stomach of the calf. 



It has been suggested that the ferment might act by inducing 

 a fermentation in the sugar of milk, giving rise to lactic acid, 

 which precipitates the casein by virtue of its being an acid. But 

 this view is disproved not only by the difference in the product 

 mentioned above, but also by the fact that casein precipitated 

 from milk by neutral salts, washed free from milk sugar and 

 redissolved, forms a fluid which is readily curdled by rennet like 

 natural milk. It seems probable that the ferment really acts on the 

 casein, converting it in some way from a soluble to an insoluble form. 



Bile. 



The quality of bile varies much, not only in different animals, 

 but in the same animal at different times. It is moreover affected 

 by the length of the sojourn in the gall-bladder ; bile taken direct 

 from the hepatic duct, especially when secreted rapidly, contains 

 little or no mucus; that taken from the gall-bladder, as of 

 slaughtered oxen or sheep, is loaded with mucus. The colour of 

 the bile of carnivorous and omnivorous animals, and of man, is a 

 bright golden red : of graminivorous animals, a golden green/ or a 

 bright green, or a dirty green, according to circumstances, being 

 much modified by retention in the gall-bladder. The reaction 

 alkaline. The following may be taken as the average composition 

 of human bile (Frerichs). 



In 1000 parts. 

 Water 8 5 9 . 2 



Solids : 



Bile Salts ... ... .., ... ... 91-4 



Fats, &c. . . . ... ... ... ... 9-2 



Cholesterin ... ... ... 2'6 



Mucus and Pigment 29*8 



Inorganic Salts ... ... ... ... 7-3 



~ 140-8 



The entire absence of proteids is a marked feature of bile. 

 With regard to the inorganic salts, the points of interest are the 

 presence of a large quantity of sodium chloride ('2 to "27 per cent.), 

 the presence of phosphates, of iron (about '006 p. c.), manganese, 

 and occasionally, at all events, of copper. The ash contains soda in 



