It is as essential to the body that its wastes be carried off from the organs, as that the used-up material be replaced by new. Not merely must matter for assimilation be provided, but the various waste products must be removed. Here again the blood-vessels and absorbents come into play. Absorbents are found not only in the walls of the alimentary canal, but all over the body. The wastes of each working tissue are passed out into them, and by them carried into the blood-vessels; these in turn carry the wastes to the lungs, kidneys, and skin, which get rid of them. The blood is thus as important in relation to removing the waste matters of an organ as in regard to supplying it with food and oxygen.

What must be carried to all parts in addition to food ? Where does the blood get oxygen? What is meant by the respiratory organs? By respiration ?

What happens when an organ works? How are oxidized tissues replaced ? What is meant by assimilation ? By dissimilation ?

What is needful to each organ in addition to a supply of fresh material?

Nutrition

From what has been said above it is clear that the nourishment of the body is a very complicated process. It implies—(1) the reception of food from outside ; (2) the digestion of food; (3) the absorption of digested food; (4) the conveyance of absorbed food to all parts by the blood; (5) the taking up of wastes from the different organs ; (6) the conveyance of these wastes by the blood to excretory organs which pass them out of the body; (7) the absorption of oxygen in the lungs, and its conveyance by the blood to every organ ; (8) assimilation or the building up of new tissue from materials brought by the blood; and (9) disassimilation, or the breaking down of working tissues by combination with oxygen.

In subsequent chapters we shall have to consider in more detail, Digestion, Circulation, Absorption, Respiration, and Excretion. The sum total of the actions of all the organs concerned in the nourishment of the body is known as the function of nutrition.

Where do we find absorbents in addition to those of the alimentary canal? What is their function? What part does the blood play in the removal of wastes?

Enumerate the processes concerned in the nourishing of the body.

What is meant by the function of nutrition?