154 MANUAL OF PHYSIOLOGY. 



applied. Being kept in motion in a large quantity of liquid 

 which dissolves the cases in which the food particles are con- 

 tained, the bolus of food soon falls asunder and each of its in- 

 gredients is fully exposed to the action of the gastric juice. The 

 acid reaction of the gastric fluid neutralizes the alkalinity of the 

 saliva, so that the action of the ptyalin is hindered, and the 

 starch granules float about quite unaffected by the pepsin or hy- 

 drochloric acid. The heat of the stomach melts the fats, and the 

 motion breaks up the oily fluid into smaller masses. They are 

 then mingled with the general fluid, which becomes more and 

 more turbid owing to the admixture of starch granules, fat glo- 

 bules, dissolved parapeptones, and minute particles of partially 

 digested proteids. This dull gray turbid fluid is called chyme. 

 The proteids (which class of food-stuffs are most profoundly 

 affected by the gastric digestion) are changed more or less rapidly 

 according as their particles are small and uncovered, or large 

 and massed together, so that they are more or less readily reached 

 by the gastric juice, and also in proportion to the facility with 

 which they form acid albumin. The chyme contains but little 

 peptone, so that we may conclude that, when formed, it is rapidly 

 absorbed as are also the soluble sugar and ordinary fluids taken 

 with the food. The chyme begins to leave the pylorus soon after 

 gastric digestion has begun, some passing into the duodenum in 

 about half an hour. The materials which resist the gastric secre- 

 tion, or are affected very slowly by it, are retained many hours in 

 the stomach, and the pylorus may refuse exit to such materials 

 for an indefinite time, so that after causing much uneasiness they 

 are finally removed by vomiting. However, many solid masses, 

 unchewed vegetables, etc., escape through the pylorus when it 

 opens to let out the chyme. 



