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 construction 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 necessary, therefore, to reduce to the liquid state all nutrients not already in that condition. _This reduction to the liquid state constitutes the digestive process_. *How Substances are Liquefied.*—While the reduction of solids to the liquid state is accomplished in some instances by heating them until they melt, they are more frequently reduced to this state by subjecting them to the action of certain liquids, called _solvents_. Through the action of the solvent the minute particles of the solid separate from each other and disappear from view. (Shown in dropping salt in water.) At the same time they mix with the solvent, forming a _solution_, from which they separate only with great difficulty. For this reason solids in solution can diffuse through porous partitions along with the solvents in which they are dissolved (page 73). By digestion the nutrients are reduced to the form of a solution. _The process is_, simply speaking, _one of dissolving_. The liquid employed as _the digestive solvent is water_. The different nutrients dissolve in water, mixing with it to form a solution which is then passed into the body proper.