DIGESTION. 249 



The gastric fluid acts as a general solvent for some of the saline con- 

 stituents of the food, as, for example, particles of common salt, which 

 may happen to have escaped solution in the saliva; while its acid may 

 enable it to dissolve some other salts which are insoluble in the latter or 

 in water. It also dissolves cane sugar, and by the aid of its mucus causes 

 its conversion in part into grape sugar. 



The action of the gastric juice in preventing and checking putrefac- 

 tion has been often directly demonstrated. Indeed, that the secretions 

 which the food meets with in the alimentary canal are antiseptic in their 

 action, is what might be anticipated, not only from the pronenessio de- 

 composition of organic matters, such as those used as food, especially 

 under the influence of warmth and moisture, but also from the well- 

 known fact that decomposing flesh (e.g., high game) may be eaten with 

 impunity, while it would certainly cause disease were it allowed to enter 

 the blood by any other route than that formed by the organs of digestion. 



Time occupied in Gastric Digestion. Under ordinary condi- 

 tions, from three to four hours may be taken as the average time occupied 

 by the digestion of a meal in the stomach. But many circumstances will 

 modify the rate of gastric digestion. The chief are: the nature of the 

 food taken and its quantity (the stomach should be fairly filled not dis- 

 tended); the time that has elapsed since the last meal, which should be 

 at least enough for the stomach to be quite clear of food; the amount of 

 exercise previous and subsequent to a ineal (gentle exercise being favor- 

 able, over-exertion injurious to digestion); the state of mind (tranquillity 

 of temper being essential, in most cases, to a quick and due digestion); 

 the bodily health; and some others. 



Movements of the Stomach. The gastric fluid is assisted in 

 accomplishing its share in digestion by the movements of the stomach. 

 In granivorous birds, for example, the contraction of the strong muscular 

 gizzard affords a necessary aid to digestion, by grinding and triturating 

 the hard seeds which constitute part of the food. But in the stomachs of 

 man and other Mammalia the motions of the muscular coat are too feeble 

 to exercise any such mechanical force on the food; neither are they 

 needed, 'for mastication has already done the mechanical work of a giz- 

 zard; and experiments have demonstrated that substances enclosed in 

 perforated tubes, and consequently protected from mechanical influence, 

 are yet digested. 



The normal actions of the muscular fibres of the human stomach 

 appear to have a threefold purpose; (1) to adapt the stomach to the 

 quantity of food in it, so that its walls may be in contact with the food 

 on all sides, and, at the same time, may exercise a certain amount of 

 compression upon it; (2) to keep the orifices of the stomach closed until 

 the food is digested; and (3) to perform certain peristaltic movements, 

 whereby the food, as it becomes chymified, is gradually propelled toward, 



