120 THE BODY AT WORK 



thought that disintegration is a necessary preliminary to the 

 rearrangement of the sub-groups. A large variety of proteins 

 is ingested as food. Many of them, especially the vegetable 

 proteins, are quite foreign to the body. By the activity of 

 pancreatic juice and erepsin, they are broken into small and 

 relatively stable groups of atoms, which are again fitted 

 together into the particular forms of protein which are of use 

 to the economy. 



The Story of a Meal. The chemistry of digestion will be 

 understood most readily if the constituents of a meal are 

 traced from their entrance into the mouth to their absorption 

 through the wall of the alimentary canal, or abandonment as 

 indigestible. 



We may describe as a typical meal one consisting of bread, 

 vegetables, cane-sugar, meat, milk, fat, and cheese. In the 

 mouth the various foods are crushed and mixed with the alka- 

 line secretions of the salivary glands. A certain amount of 

 the cooked starch contained in the bread is changed into 

 maltose. In the stomach the digestion of starch is continued 

 for a time, but a large part even of the cooked starch awaits 

 the action of pancreatic juice. A certain amount of cane-sugar 

 is converted into dextrose and levulose, which are rapidly 

 absorbed into the blood ; but this action is due to hydrochloric 

 acid, and probably affects a comparatively small part of the 

 cane-sugar swallowed. Fat is quite unaltered in the stomach. 

 All proteins are attacked by pepsin, but some yield to digestion 

 more readily than others, Gluten of bread, like all vegetable 

 proteins, is comparatively resistant ; but since it is presented 

 to the action of pepsin in small quantities and in a spongy 

 form very suitable for digestion it is probable that most of 

 it is peptonized in the stomach. Chemists experimenting 

 with gastric juice taken from the stomach, and reproducing 

 the conditions as to temperature, removal of products of action, 

 etc., as closely as it is possible to reproduce them in the labora- 

 tory, find that the various foods take different times to digest. 

 The proteins of meat are more quickly peptonized when raw 

 than after coagulation by heat. The same is true of white of 

 egg. Amongst different varieties of cooked flesh, beef is more 

 quickly peptonized than fish. The casein of milk is more 

 quickly peptonized than any other protein ; and it also is no 



