CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 431 



But even at the best, its actions are slow and feeble. Moreover 

 many observers have obtained negative results, so that the various 

 statements are conflicting. Besides, we have no exact knowledge 

 as to the amount to which such a secretion takes place under 

 normal circumstances in the living body. We may therefore 

 conclude that, at present at all events, we have no satisfactory 

 reasons for supposing that the actual digestion of food in the 

 intestine is, to any great extent, aided by such a juice. 



Of the possible action of other secretions of the alimentary 

 canal, as of the caecum and large intestine, we shall speak when 

 we come to consider the changes in the alimentary canal. 



251. Gallstones. Concretions, often of considerable size, 

 known as gallstones are not unfrequently formed in the gall 

 bladder, and smaller concretions are sometimes formed in the 

 bile passages. In man two kinds of gallstones are common. One 

 kind consists almost entirely of cholesterin, sometimes nearly free 

 from any admixture with pigment, sometimes more or less dis- 

 coloured with pigment. Gallstones of this kind have a crystalline 

 structure, and when broken or cut shew frequently radiate and 

 concentric markings. The other kind consists chiefly of bilirubin 

 in combination with calcium. Gallstones of this kind are dark 

 coloured and amorphous. Less common than the above are small 

 dark coloured stones, having often a mulberry shape, consisting 

 not of bilirubin itself, but of one or other derivative of bilirubin. 

 Gallstones consisting almost entirely of inorganic salts, calcic 

 carbonates and phosphates, are also occasionally met with. In the 

 lower animals, in oxen for instance, bilirubin gallstones are not 

 uncommon, but cholesterin gallstones are rare. 



A gallstone appears always to contain a more or less obvious 

 'nucleus,' around which the material of the stone has been de- 

 posited, and which may be regarded as the origin of the stone ; 

 the real cause of the formation of the stone lies however in certain 

 changes in the bile, by which the cholesterin, or bilirubin, or other 

 constituent ceases to remain dissolved in the bile. But we cannot 

 discuss this matter here. 



