286 THE TISSUES AND MECHANISMS OF DIGESTION. 



Of the possible action of other secretions of the alimentary canal, as of 

 the csecum and large intestine, we shall speak when we come to consider the 

 changes in the alimentary canal. 



220. Gall-stones. Concretions, often of considerable size, known as gall- 

 stones, are not unfrequently formed in the gall-bladder, and smaller concre- 

 tions are sometimes formed in the bile passages. In man two kinds of 

 gall-stones are common. One kind consists almost entirely of cholesterin, 

 sometimes nearly free from any admixture with pigment, sometimes more or 

 less discolored with pigment. Gall-stones of this kind have a crystalline 

 structure, and when broken or cut show frequently radiate and concentric 

 markings. The other kind consists chiefly of bilirubin in combination with 

 calcium. Gall-stones of this kind are dark-colored and amorphous. Less 

 common than the above are small, dark-colored stones, having often a mul- 

 berry shape, consisting not of bilirubin itself, but of one or other derivative 

 of bilirubin. Gall-stones consisting almost entirely of inorganic salts, calcic 

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

 animals, in oxen for instance, bilirubin gall-stones are not uncommon, but 

 cholesterin gall-stones are rare. 



A gall-stone appears always to contain a more or less obvious "nucleus," 

 around which the material of the stone has been deposited, 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. 



THE SECRETION OF PANCREATIC JUICE AND OF BILE. 



221. The secretion of pancreatic juice. Although in some cases, as 

 that of the parotid of the sheep, the flow of saliva is continuous or nearly so, 

 in most animals, as in man, the intermittence of the secretion is very nearly 

 absolute. While food is in the mouth saliva flows freely, but between meals 

 only just sufficient is secreted to keep the mouth moist, and probably the 

 greater part of this is supplied not by the larger salivary but by the small 

 buccal glands. The flow of pancreatic juice, on the other hand, is much 

 more prolonged, being in the rabbit continuous, and in the dog lasting for 

 twenty hours after food. But this contrast between the secretion of saliva 

 and that of pancreatic juice is natural, since the stay of food in the mouth, 

 even during a protracted feast, is relatively short, whereas the time during 

 which the material of a meal is able in some way or other to affect the pan- 

 creas is very prolonged. 



The flow, though continuous or nearly so, is not uniform. In the dog the 

 flow of pancreatic juice begins immediately after food has been taken, and 

 rises to a maximum which may be reached within the first or, as in the case 

 furnishing the diagram given in Fig. 88, the second hour, but which more 

 commonly is not reached until the third or fourth hour. This rise is then 

 followed by a fall, after which there is a secondary rise, reaching a second 

 maximum at a very variable time, but generally between the fifth and 

 seventh hours. This second maximum, however, is never so high as the 

 first. 



The second rise may be due to material absorbed from the intestines 

 being carried in the circulation to the pancreas, and so directly exciting the 

 gland to activity, much in the same way as, in the case of the stomach, the 

 absorption of digested material promotes the flow of gastric juice (see 202) ; 

 and a similar absorption may contribute to the first rise also, but it is more 

 probable that so marked and sudden a rise as this is carried out by some 



