352 A MANUAL OF PHYSIOLOGY 



the total acidity as determined by titration. Then comes a 

 stage where the hydrochloric acid has so much increased 

 that, after combining with all the proteids, some of it 

 remains over as free acid. The lactic acid now rapidly dis- 

 appears from the stomach because the growth of the bacteria 

 that produce it is checked by the hydrochloric acid ; and 

 after a time the total acidity begins to fall, the fully-digested 

 proteids being continually absorbed in the form of peptones, 

 which are only found in traces, if at all, in the chyme. This 

 fall continues till the third or fourth hour, the proportion ot 

 free to combined acid continuing, nevertheless, to rise, since 

 nearly all that is now secreted remains free. Easily-diffusible 

 bodies, such as sugars and some of the organic crystalline 

 constituents of meat, e.g., kreatin, will also pass through 

 the gastric mucous membrane into the blood. 



The substances which reach the duodenum are : (i) the 

 whole of the fats, with no chemical and little physical change. 

 But the partial digestion in the stomach of the envelopes 

 and protoplasm of the cells of adipose tissue, and of the 

 proteid which keeps the fat of milk in emulsion, prepares 

 the fats for what is to follow in the intestine. (2) All 

 the proteids which have not been carried to the stage of 

 peptone, and perhaps some peptone. (3) All the starch 

 and dextrins and glycogen, if any be present which have 

 not been converted into maltose, and possibly a little 

 maltose. (4) Elastin, nuclein, cellulose, and other sub- 

 stances not digestible or digestible only with difficulty in 

 gastric juice. (5) The constituents of the gastric juice 

 itself, including pepsin. The ptyalin of the saliva has been 

 already digested and destroyed. 



It must be remembered that all this time, even from the 

 beginning of digestion, a certain amount of pancreatic juice 

 has been finding its way into the duodenum in response to 

 the presence of food in the stomach acting through the reflex 

 mechanism we have described. The secretion of bile, too, 

 always going on, has quickened its pace, and the gall-bladder 

 is getting more and more full as the meal proceeds and 

 gastric digestion begins. When the acid chyme, a grayish 

 liquid, turbid with the debris of animal and vegetable tissues 



