CHAP, i.] DIGESTION. 293 



reduced in acidity, by the addition of saliva, causes a flow of 

 gastric juice. This, already commencing while the food is as yet 

 in the mouth, increases as the food accumulates in the stomach, 

 and as, by the churning gastric movements, unchanged particles 

 are continually being brought into contact with the mucous 

 membrane. Moreover, the absorption of the earlier digested 

 portions gives rise to a further increase of secretion and especially 

 of pepsin. The percentage of pepsin in the gastric juice (in 

 the dog) varies considerably, actually sinking during the earlier 

 stages but rising rapidly afterwards and attaining a maximum 

 at about the fourth or fifth hour. The secretion of acid appears 

 to continue at a fairly constant rate; and consequently, unless 

 neutralized by fresh alkaline food, the reaction of the gastric 

 contents becomes more and more distinctly acid as digestion 

 proceeds. It would appear that in man, sometimes at least, the 

 contents of the stomach do not at first contain any free acid and 

 during this period the conversion of starch into sugar can still 

 go on. When the contents become acid, the conversion is arrested, 

 and indeed the amylolytic ferment probably destroyed. The fats 

 themselves probably remain in great measure unchanged ; though 

 it would appear that in the dog at least a certain amount of fat 

 can be digested, that is emulsionized, or even partly split up 

 into fatty acids, by the action of the gastric juice, and absorbed. 

 Moreover even in man, through the conversion of proteids into 

 peptone, not only are the more distinctly proteid articles of food, 

 such as meat, broken up and dissolved, but the proteid framework, 

 in which the starch and fats are frequently imbedded, is loosened, 

 the starch-granules are set free, and the fats, melted for the most 

 part by the heat of the stomach, tend to run together in large 

 drops, which in turn are more or less apt to be broken up into 

 an imperfect emulsion. The collagenous tissues are dissolved; 

 and hence the natural bundles of meat and vegetables fall asunder; 

 the muscular fibre splits up into discs, and the protoplasm is 

 dissolved from the vegetable cells. Milk is at once curdled by the 

 rennet ferment and the clotted casein subsequently dissolved. 

 Since peptone and the other products of artificial digestion with 

 gastric juice have been found in the contents of the stomach, 

 we have every reason to believe that natural digestion in the 

 stomach agrees with the results of laboratory experiments described 

 in a previous section. "While these changes are proceeding, the 

 thick turbid greyish liquid or chyme, formed by the imperfectly 

 dissolved food, is from time to time ejected through the pylorus, 

 accompanied by even large morsels of solid less-digested matter. 

 This may occur within a few minutes of food having been taken; 

 but the larger escape from the stomach probably does not in man 

 begin till from one to two, and lasts from four to five hours, after 

 the meal, becoming more rapid towards the end, and such pieces 

 as most resist the gastric juice being the last to leave the stomach. 



