THE TISSUES AND MECHANISMS OF DIGESTION. 



forth by a reflex act. Yet it is stated that, unlike the case of saliva, the 

 secretion of pancreatic juice continues after all the nerves going to the gland 

 have been divided, an operation which would do away with the possibility 

 of reflex action. Such an experiment, however, cannot be regarded as 

 decisive, since it is almost impossible to be sure of dividing all the nerves. 



No evidence has yet been brought forward to prove the existence of any 

 double nervous mechanism similar to that of chorda fibres and sympathetic 

 fibres in the salivary gland. All that can be said is that, when the gland is 

 stimulated to secrete, the bloodvessels are dilated as in the salivary gland ; 

 and we have already (203) dwelt on the histological changes which accom- 

 pany secretion. We may add that when the gland is stimulated to increased 

 secretion, the increase is not merely an increase of water; the discharge of 

 solids is increased even more than the discharge of water, so that the per- 

 centage of solids in the juice increases. 



The quantity of pancreatic juice secreted, in the case of man, in twenty- 

 four hours has been calculated at 300 c.c., but such a calculation is of very 

 uncertain value. 



We have seen ( 197) that in the salivary glands the pressure which may 

 be exerted by the fluid in the ducts is very considerable, exceeding it may 

 be even the blood-pressure in the carotid artery. In this respect the pan- 

 creas differs from the salivary glands. When] in a rabbit, a can u la con- 

 nected with a vertical tube or a manometer is placed in the pancreatic duct, 

 the column of fluid does not rise above a height corresponding to a pressure 

 of about 17 mm. of mercury. But at this pressure the gland becomes cedem- 

 atous on account of the juice secreted passing back through the walls of 

 the ducts and alveoli into the connective tissue ; a much higher pressure is 

 needed to render a salivary gland cedematous ; and whether the IOW T pressure 

 observed in the pancreas is due to the ease with which oedema takes place, 

 or to the actual secretion not being able to reach a higher pressure, cannot 

 be stated with certainty. 



222. The secretion of bile. The act of secretion of bile by the liver 

 must not be confounded with the discharge of bile from the bile-duct into 

 the duodenum. When the acid contents of the stomach are poured over 

 the orifice of the biliary duct, a gush of bile takes place. Indeed, stimu- 

 lation of this region of the duodenum with a dilute acid at once calls 

 forth a flow, though alkaline fluids so applied have little or no effect. 

 When no such acid fluid is passing into the duodenum no bile is, under 

 normal circumstances, discharged into the intestine. The discharge is due 

 to a contraction of the muscular walls of the gall-bladder and ducts, accom- 

 panied by a relaxation of the sphincter of the orifice ; both acts are prob- 

 ably of a reflex nature, but the details of the mechanism have not been 

 worked out. 



The secretion of bile, "on the other hand, as shown by the result of bili- 

 ary fistulse, is continuous ; it appears never to cease. When no food is 

 taken the bile passes from the liver along the hepatic and then back along 

 the cystic duct (the flow being aided probably by peristaltic contractions 

 of the muscular fibres of the duct) to the gall-bladder, where it is tem- 

 porarily stored ; hence in starving animals, when no discharge is excited by 

 food, the gall-bladder becomes greatly distended with bile. But the secre- 

 tion, though continuous, is not uniform. The rate of secretion varies, and 

 is especially influenced by food ; it is seen to rise rapidly after meals, 

 reaching its maximum, in dogs, in from four to eight hours. There seems 

 to be an immediate, sudden rise when food is taken, then a fall, followed sub- 

 sequently by a more gradual rise up to the maximum, and ending in a final 

 fall to the lowest point. The curve of secretion, in fact, resembles that of 



