DIGESTION 177 



or sarcous substance, consisting largely of myosin, undergoes a corresponding 

 change. If the quantity of meat be not too large and the gastric juice be 

 secreted in proper amount, it is possible that all the meat will be digested in 

 the stomach. It is quite probable, however, that this is not the case and that 

 a portion of the semidigested meat passes into the intestine, where its final 

 solution is effected. 



The white of egg, especially when slightly boiled, is much more readily 

 digested than when raw or firmly coagulated by prolonged boiling. In either 

 condition, however, the supporting tissue is dissolved and peptonized, after 

 which the native albumin undergoes the same change. The yolk of the egg 

 consists largely of fat held in suspension by a protein substance, vitellin, 

 which is also capable of transformation into peptone. 



Adipose tissue is similarly reduced. The protein of the connective tissue 

 and of the fat vesicles is dissolved and peptonized and the fat-drops set 

 free. 



Milk undergoes a peculiar change in composition before its chief protein 

 constituent, caseinogen, can be transformed into peptone. The caseinogen 

 in the presence of calcium salts is always in the soluble state. When acted 

 on by the gastric juice, the caseinogen undergoes a chemic change by reason 

 of which it combines with calcium salts and is then transformed into a solid 

 compound casein. This change is due to the presence and activity of the 

 enzyme, rennin. The necessity for this change in the process of digestion, 

 however, is not apparent. The coagulated casein presents itself in the form 

 of a flocculent curd, which is finer in human than in cow's milk, and hence 

 more easily digestible. After its production, the casein is acidified by the 

 hydrochloric acid and then converted by the pepsin into peptone. 



Vegetables, though consisting of a woody or cellulose framework, undergo 

 a partial disintegration in the stomach. When they are boiled and dis- 

 integrated by the teeth, the gastric juice is enabled to penetrate the frame- 

 work and dissolve and peptonize the various protein constituents. As a 

 general rule, the vegetable proteins are more difficult of digestion than the 

 animal proteins. 



Duration of Gastric Digestion. The time required for the stomach 

 to digest and discharge any given article of food has been shown by 

 Cannon to depend partly on its chemic composition and partly on its 

 capacity for absorbing hydrochloric acid. From an examination of the 

 stomach and duodenum of the cat by means of Rontgen rays and the 

 fluoroscopic screen, after the administration of equal quantities, 25 c.c., 

 of pure protein, fat, and carbohydrate, mixed with 5 grams of bismuth, 

 it became possible to determine the rate at which they left the stomach 

 from the length of the food masses in the duodenum and small intestine 

 as indicated by the shadows on the screen, at intervals of half an hour or 

 longer. The duration of the observations extended over a period of seven 

 hours. 



When a pure protein, e.g., boiled beef free from fat, boiled haddock, or 

 the white meat of fowls is administered, foods which not only excite the 

 flow of gastric juice but readily absorb hydrochloric acid, the pylorus remains 

 closed for some time, scarcely any protein leaving the stomach during the first 

 half hour. Shortly after this when free hydrochloric acid makes its appear- 

 ance, the signal for the relaxation of the sphincter, the pylorus opens from 



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