322 A YEAR IN SCIENCE 



by the blood. The walls of the alimentary canal have 

 no openings, neither have the walls of the blood vessels. 

 Consequently the food must pass through these walls 

 by diffusion. Obviously, then, the purpose of the process 

 of digestion is to act upon the food in such a way that 

 it can diffuse through these membranes. This means 

 that it must be not only in the form of a liquid but in 

 the form of particular kinds of liquids, for all liquids 

 do not diffuse. 



Action upon food. In its passage through the canal, 

 food is acted upon both physically and chemically. 

 The food is chewed or rubbed into small fragments. 

 If soluble, these then dissolve. If they can not be disr 

 solved, they then undergo chemical changes which 

 make them soluble and simpler in their composition. 

 This action is brought about by substances in- the 

 digestive juices called enzymes. 



Digestion in mouth. When the food enters the 

 mouth it is first chewed or masticated. This enables 

 us to swallow it more easily. But, of far greater 

 importance is the fact that by breaking the food up 

 into small particles we expose more of it to the action 

 of the digestive juices. It is very important that we 

 chew our food thoroughly. "In chewing, the food is 

 moved about largely by the action of the tongue. At 

 the same time, it is mixed with the liquid in the 

 mouth, which is a mixture of mucus and saliva. 



Saliva is a slightly alkaline liquid, mainly water. By 

 jt the food is moistened and softened ; at the same time 



