DIGESTION. 165 



introduction. Three hours is probably sufficient, as a rule, for complete 

 stomach digestion, in man, when the food is in moderate quantity and has 

 been properly prepared by cooking and mastication. In the dog, where 

 the food is generally swallowed in fragments of some size, the process is 

 longer ; and a moderate meal of fresh uncooked meat requires from nine 

 to twelve hours for its liquefaction and disappearance from the stomach. 



The gastric juice, after accomplishing its work in digestion, is reab- 

 sorbed from the alimentary canal by the blood-vessels. It forms a [ 

 vehicle for the dissolved nutritious material, and again enters the 

 circulation, bringing with it the alimentary substances in solution. 

 An abundant supply of the secretion may therefore be poured out 

 during digestion, at an expense to the blood, at any one time, of only 

 a small quantity of fluid. The simplest investigation shows that neither 

 gastric juice nor peptones accumulate to any considerable amount in the 

 stomach ; each portion of the food, when digested, being disposed of by 

 absorption, together with its solvent fluid. There must be accordingly, 

 during digestion, a continuous circulation of fluids from the blood-ves- 

 sels to the alimentary canal, and from the alimentary canal back again 

 to the blood-vessels. 



That this really takes place is shown by the following facts : First, 

 if a dog be killed some hours after feeding, there is never more than a 

 very small quantity of fluid found in the stomach, just sufficient to 

 smear over and penetrate the half digested pieces of meat; and sec- 

 ondly, in the living animal, gastric juice, drawn from the fistula five 

 or six hours after digestion has been going on, contains little or no more 

 peptone in solution than that extracted fifteen or twenty minutes after 

 the introduction of food. To obtain gastric juice saturated with ali- 

 mentary matter, it must be artificially digested with food in test-tubes, 

 where absorption and renovation cannot take place. 



The secretion of gastric juice is much influenced by nervous condi- 

 tions. It was noticed by Beaumont, in his experiments with St. Mar- 

 tin, that irritation of the temper or other moral causes would often 

 dimmish or suspend the supply of the gastric fluids. Any febrile 

 action or unusual fatigue would exert a similar effect. Every one is 

 aware how readily mental disturbances, such as anxiety, anger, or vexa- 

 tion, will take away the appetite and interfere with digestion. Impres- 

 sions of this kind, especially at the commencement of the process, seem 

 liable to produce a lasting effect and to disturb digestion for the entire 

 day. In order, therefore, that the function may be properly performed, 

 food should be taken only when the appetite demands it ; it should be 

 thoroughly masticated ; and, finally, both mind and body, particularly 

 in the early part of digestion, should be free from unusual or dis- 

 agreeable excitement. 



Pancreatic Juice, 



The pancreas, which is similar in general structure to the salivary 

 glands, lies across the upper part of the abdomen, with its larger or 



