DIGESTION. 189 



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 connective tissue is dis- 

 solved and peptonized, after which the native albumin undergoes the 

 same change. The yolk of the egg consists largely of fat held in sus- 

 pension by a proteid substance, vitellin, which is also capable of 

 transformation into peptone. 



Adipose tissue is similarly reduced. The proteids of the con- 

 nective tissue and of the fat vesicles are dissolved and peptonized 

 and the fat-drops set free. Milk undergoes a peculiar change in 

 composition before its proteid constituents can be transformed into 

 peptones. The caseinogen in the presence of calcium salts is always 

 in the soluble state. When acted on by the gastric juice, the case- 

 inogen undergoes coagulation which consists in the formation of a 

 solid compound, casein, and a soluble albumin. This change is due 

 to the presence and activity of the ferment rennin. The necessity 

 for this change, however, is not apparent. The coagulated casein 

 presents itself in the form of a flocculcnt curd, which is finer in human 

 than in cow's milk, and hence more easily digestible. The casein 

 is acidified by the hydrochloric acid and then converted into peptone. 



Vegetables, though consisting of a woody or cellulose framework, 

 undergo a partial disintegration in the stomach. When boiled and 

 physically disintegrated by the teeth, the gastric juice is enabled to 

 penetrate the framework and dissolve and peptonize the various 

 proteid constituents^ As a general rule, -the vegetable^ proteids are \JK/ 

 more difficult of digestion than the animal proteids^ /)(\ 



Duration of Gastric Digestion. The length of time the food 

 remains in the stomach and the relative digestibility of different 

 articles of food were carefully studied by Dr. Beaumont on St. Martin, 

 and though the results obtained by him may not be absolutely correct, 

 viewed in the light of recent knowledge of the digestive process, yet 

 in the main they have been corroborated in various ways. As a 

 result of many observations Dr. Beaumont came to the conclusion 

 that the average length of time an ordinary meal consisting of meat, 

 bread, potatoes, etc., remained in the stomach undergoing digestion 

 was about three and a rialf hours^ the duration of the process, how- 

 ever, being increased when an excessive quantity of food was taken or 

 the quantity and quality of the gastric juice impaired by abnormal 

 conditions of the system. As soon as the food is liquefied by the 

 gastric juice that portion not absorbed by the gastric vessels passes 

 into the intestines, this continuing for two to three hours until the 

 stomach is completely emptied. The relative digestibility of the dif- 

 ferent foods was also made the subject of many experiments by Dr. 

 Beaumont. After repeating and verifying his observations made 

 under varying conditions, he summed up his results in a table, of 



