90 DIGESTION 



the famine degeneration be arrested. Carnivorous animals 

 eat meat, which contains proteins and fats. The proteins 

 and the fats cannot pass through the walls of the intestine 

 until they have undergone a change. Animals that are 

 herbivorous consume a great quantity of starch and sugar, 

 and with the starch is always more or less protein matter. 

 Starch is again insoluble in water. 



Lining the alimentary canal is a membrane, and it is only 

 certain substances that can diffuse through this membrane. 

 Sugar is soluble in water, and can pass through it, leaving the 

 cavity of the intestine and arriving in the blood-vessels which 

 run in its wall. Besides the foods mentioned above we take 

 up a certain amount of salts and a certain amount of other 

 mineral substances, most of which are capable of passing in 

 solution through the membrane. We live then on protein 

 matter, fat, starch or sugar, with a certain amount of other 

 minerals and water. We must have proteins, for they alone 

 contain nitrogen, and nitrogen is wanted to build up the new 

 proteins formed in the body. We may fill our stomach with 

 starches, proteins and fats, but we should starve unless these 

 foodstuffs were rendered soluble. Until this is done the food 

 is of no more use to the body in the alimentary canal than it 

 was before it was swallowed. 



But mth the meat and the fat and the starch and the 

 sugar there are a number of other substances which form, 

 as it were, the packing of these foodstuffs. They are useless as 

 food, and when once the food is dissolved out from them they 

 pass away from the body as utterly indigestible. They pass 

 away without being dissolved into the blood and have never 

 formed any real part of the body at all. It seems that for 

 the proper action of the intestine a certain amount of in- 

 digestible food such as cellulose, i.e. the coating of vegetable 

 cells, and the tougher fibres of meat, is necessary in order 

 to supply a certain amount of bulk without which the normal 

 action of the intestine is liable to stagnate. 



Digestion 



Digestion is the act of preparing and dissolving the food- 

 stuffs so that they may pass through the membranous wall 



