294 CHANGES OF FOOD IN THE STOMACH. [BOOK n. 



The time 'taken up in gastric digestion probably varies not only 

 \vith 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, 

 while the stomachs of rabbits are never empty but always remain 

 largely filled with food. 



In a dog fed on an exclusively meat diet, nearly the whole of 

 the digestion is carried out by the stomach, very little work 

 apparently being left for the intestines. In man, especially on 

 a mixed diet, the case in all probability is different, a considerable 

 portion of the proteids as well as the greater part of the fats and 

 carbohydrates passing but little changed through the pylorus. 

 But our information on this matter is imperfect being chiefly 

 drawn from the study of cases of gastric or duodenal fistula, in 

 which probably the order of things is not normal or being in 

 large measure deductions from experiments on dogs, whose economy 

 in this respect must be largely different from our own. 



In the presence of healthy gastric juice, and in the absence of 

 any nervous interference, the question of the digestibility of any 

 food is determined chiefly by mechanical conditions. The more 

 finely divided the material, and the less the proteid constituents 

 are sheltered by not easily soluble envelopes, such as those of 

 cellulose, the more rapid the solution. So also pieces of hard- 

 boiled egg, which have to be gradually dissolved from the outside, 

 are less easily digested than the more friable muscular fibre, the 

 repeated transverse cleavage of which increases the surface exposed 

 to the juice. Unboiled white of egg again, unless thoroughly 

 beaten up and mixed with air, is less digestible than the same 

 boiled. The unboiled white forms a viscid clotted mass, of low 

 diffusibility, into which the juice permeates with the greatest 

 difficulty. And so with the other instances. Beyond this me- 

 chanical aspect of digestibility, it is to be remembered that 

 different substances may differently affect the gastric membrane, 

 promoting or checking the secretion of the juice. Hence a 

 substance, the mass of which is readily dissolved by gastric juice, 

 and which offers no mechanical obstacles to digestion, may yet 

 prove indigestible by so affecting the gastric membrane through 

 some special constituent (or possibly in other ways) as to inhibit 

 the secretion of the juice. 



That substances can be absorbed from the cavity of the stomach 

 into the circulation is proved by the fact that food when introduced 

 disappears very largely from the stomach of an animal, the pylorus 

 of which has been ligatured. But we cannot speak with certainty 

 as to what extent in ordinary life gastric absorption takes place, or 

 by what mechanism it is carried out. The presumption is, that 

 peptone and the diffusible sugars pass by osmosis direct into the 

 capillaries, and so into the gastric veins. In a dog fed on meat the 

 quantity of peptone present at any one time in the stomach has 



