I30 THE VITAL PROCESSES 



balance, or diseased. Not only are drugs of no value in 

 health, but their use is liable to do much harm. 



NATURE OF DIGESTION 



Before the nutrients can be oxidized at the cells, or built 

 into the protoplasm, they undergo a number of changes. 

 These are necessary for their entrance into the body, for 

 their distribution by the blood and the lymph, and for the 

 purposes which they finally serve. The first of these 

 changes is preparatory to the entrance of the nutrients and 

 is known as digestion. The organs which bring about 

 this change, called digestive organs, have a special con- 

 struction which adapts them to their work. It will assist 

 materially in understanding these organs if we first learn 

 something of the nature of the work which they have to 

 perform. 



How the Nutrients get into the Body. The nature of 

 digestion is determined by the conditions affecting the 

 entrance of nutrients into the body. Food in the stomach 

 and air in the lungs, although surrounded by the body, are 

 still outside of what is called the body proper. To gain 

 entrance into the body proper, a substance must pass 

 through the body wall. This consists of the skin on the 

 outside and of the mucous linings of the air passages and 

 other tubes and cavities which are connected with the 

 external surface. 



To get from the digestive organs into the blood, the 

 nutrients must pass through the mucous membrane lining 

 these organs and also the walls of blood or lymph vessels. 

 Only liquid materials can make this passage. It is neces- 

 sary, therefore, to reduce to the liquid state all nutrients 

 not already in that condition. This reduction to the liquid 

 state constitutes the digestive process. 



