CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 389 



is dissolved in the stomach, the whole tissue is softened and to 

 a certain extent disintegrated. Milk is curdled and the curd 

 subsequently more or less dissolved. 



The thick soup-like acid chyme consists accordingly partly of 

 substances which have entered into actual solution, partly of 

 mere particles or droplets of proteid, fatty or other nature and 

 partly of masses small or great, which may be recognized under 

 the microscope as more or less changed portions of animal or 

 vegetable tissue. The amount of material actually dissolved 

 is in most specimens of chyme exceedingly small. When the 

 solid parts are removed by nitration the clear nitrate contains 

 besides salts, pepsin and free hydrochloric acid (the constituents 

 of the gastric juice), a small amount of sugar, of some of the 

 bye products of proteid digestion, and of albumose and peptone. 

 The sugar is often absent, and the amount of peptone (or 

 albumose) is always small. 



During gastric digestion the chyme thus formed 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, becom- 

 ing more rapid towards the end, and such pieces as are the 

 least broken up by the gastric juice and movements being the 

 last to leave the stomach. Water taken by itself appears to 

 be passed on at once into the small intestine. 



The time taken up in gastric digestion probably varies in the 

 same animal not only with different articles of food but also 

 with varying conditions of the stomach and of the body at large. 

 In different animals it varies very considerably, being from 12 

 to 24 hours in the dog after a full meal, while the stomachs of 

 rabbits are never empty but always remain largely filled with 

 food, even during starvation. In man the stomach probably 

 becomes empty between the usual meals. 



The total amount of change which the food undergoes in the 

 stomach, that is the share taken by the stomach in the whole 

 work of digestion, seems to vary largely in different animals, 

 and in the same animal differs according to the nature of the 

 meal. In a dog fed on an exclusively meat diet, a very large 

 part of the digestion is said to be carried out by the stomach, 

 very little work apparently being left for the intestines ; that is 

 to say, the larger part of the meal is reduced in the stomach to 

 actual solution and a considerable quantity is probably absorbed 

 directly from the stomach. In such cases the amount of pep- 

 tone found in the stomach during the digestion of the meal is 

 found to be fairly constant, from which it may be inferred that 

 the peptone is absorbed so soon as it is formed. There is also 

 evidence that fat may to a certain extent undergo in the stom- 



