IX NUTRITION 149 



as metabolism. These processes are illustrated in the 

 diagram (Fig. 23, p. 91), which should be constantly 

 consulted in connection with the following summary. 



All parts of the body are placed in communication with 

 one another by means of the bloodvessels, through which a 

 constant stream of blood is flowing in a definite direction. 



In all parts waste of substance (destructive metabolism) is 

 continually going on, and the waste products, water, carbon 

 dioxide, and some nitrogenous substance which ultimately 

 takes the form of urea — are passed either directly into the 

 blood, or first into the lymph and thence into the blood. 



At the same time the tissues withdraw nutrient materials 

 from the blood, whereby the waste of substance is made 

 good, and the cells and other elements adequately nourished 

 {constructive metabolism). Oxygen is also withdrawn from 

 the blood ; like the air supplied to a fire it is essential to the 

 oxidation or low temperature combustion with which the 

 waste of the tissues is associated. By the withdrawal of its 

 oxygen the haemoglobin of the blood alters its colour from 

 scarlet to purple. 



Thus the blood as it passes through the body is constantly 

 being impoverished by the withdrawal of nutrient matters 

 and of oxygen and as constantly being fouled by the 

 discharge into it of waste products. It reaches the capil- 

 laries of an organ as bright red, aerated blood, and leaves 

 it as purple, non-aerated blood. 



These changes, by which the blood loses nutrient matters 

 and oxygen, and gains waste products, takes place all over 

 the body. The converse processes by which nutrient matters 

 and oxygen are absorbed and waste products got rid of are 

 carried on in certain definite portions of the circulatory 

 system. 



In the walls of the enteric canal (Fig. 23, Ent. C), the 



