IX.] THE PROCESS OF DIGESTION 173 



such as sugars and fatty acids that are capable of 

 absorption by the blood, together with gases like 

 methane, and to these bacteria is ascribed the main part 

 of the work of digesting cellulose and similar carbo- 

 hydrates. 



All carbohydrates — sugars, starches, or soluble cellu- 

 loses — act as fuel for the body: having reached the 

 blood, they are passed on to the living cells, and there 

 burnt to carbon dioxide and water in order to supply 

 energy. When more carbohydrates are supplied than 

 the animal requires, they can be built up into fats either 

 for storage or for milk production, carbohydrates them- 

 selves being never stored in the animal body except in 

 the liver, which contains one special body of this nature 

 — glycogen. The combustion of the carbohydrates is 

 not, however, always so complete as that of the fats ; a 

 certain proportion, especially of the cellulose and fibre, 

 is excreted in a unoxidised form as methane or marsh- 

 gas, and to this extent the value of the carbohydrate as 

 fuel has been reduced. 



The digestion of the proteins is a more complex 

 process. The stomach secretes a gastric juice which is 

 acid and also contains an enzyme called pepsin, by 

 which the proteins are first attacked. Then in the 

 small intestine the partly digested food is mixed with 

 the pancreatic juice, which possesses an alkaline reaction 

 and generates a second enzyme called trypsin, which 

 also attacks the proteins. Bo± of these enzymes break 

 down the proteins into peptones, albumoses, and succes- 

 sively more soluble amino-acids, which are able to 

 diffuse through the walls of the intestine. There the 

 simple nitrogen compounds are once more built up into 

 proteins, and in that form are passed into the blood and 

 led to the different parts of the body ; though another 

 view is that the soluble nitrogen compounds enter the 



