DIGESTION 1 77 



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 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 corrobo- 

 rated 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 under- 

 going digestion was about three and a half hours, the duration of the proc- 

 ess, however, 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 intes- 

 tines, this continuing for two to three hours until the stomach is completely 

 emptied. The relative digestibility of the different 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 which the following is an abstract, in which the 

 mode of preparation and the time required for the digestion of different 

 foods are exhibited: 



The time required for the stomach to discharge any given article o 

 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 . 

 rays and the fluoroscopic screen, after the administration of equal quantities, 



