CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 497 



insufficient to produce any marked conversion of the starch it may 

 contain. During the rapid transit through the oesophagus no 

 appreciable change takes place. 



The amount of absorption of digested material, or even of 

 simple water from the mouth or oesophagus, must always be 

 insignificant. 



The Changes in the Stomach. 



279. The arrival of the food, the reaction of which is either 

 naturally alkaline, or is made alkaline, or at least is 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, one part after another of the food is 

 brought into contact with the mucous membrane. 



The characters of the juice appear to change somewhat as the 

 act of digestion proceeds. The amount of pepsin in the gastric 

 contents increases for some time after food is taken, and probably 

 the actual secretion increases also. The acidity of the gastric 

 contents is at first very feeble ; indeed in man, in some cases at 

 least, for some little time after the beginning of a meal no free 

 acid is present, and during this period the conversion of starch into 

 sugar may continue. This condition however is temporary only; 

 very soon the contents become acid, arresting the action of and 

 ultimately destroying the amylolytic ferment; and, since the rate 

 of the secretion of acid appears to be fairly constant, the contents 

 of the stomach, unless fresh alkaline food be taken, become more 

 acid as digestion goes on. 



The gross effect of gastric digestion is to break up and partly 

 to dissolve the larger lumps of masticated food into a thick 

 greyish soup-like liquid called chyme, with which are still mixed 

 in variable quantity larger and smaller masses of less changed 

 food. This is the result, partly of the solution of proteid matters, 

 partly of the solution of the gelatiniferous connective-tissue 

 holding the proteid elements together. In a fragment of meat, 

 for instance, the muscular fibres, through the solution of the 

 connective-tissue binding them together, fall asunder, the sarco- 

 lemma is dissolved, and the fibres themselves split up sometimes 

 longitudinally but most frequently by transverse cleavage into 

 discs, and are ultimately more or less reduced, partly into a 

 granular mass, partly to actual solution. In a piece of tissue con- 

 taining fat, the connective-tissue binding the fat cells together and 

 the envelopes of the fat cells are dissolved, so that the fat, fluid at 

 the temperature, of the body, is set free from the individual cells 

 and runs together into larger and smaller masses. In vegetable 

 tissue the proteid elements are in part dissolved and, though there 

 is no evidence that in man cellulose is dissolved in the stomach, 



