ix NUTRITION 149 



of 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 blood-vessels, 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 cells withdraw nutrient materials 

 from the blood, and assimilate them, i.e., form new living sub- 

 stance, whereby the waste is made good, and the tissues 

 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 withdrawn from the blood ; like the air supplied to a fire, it is 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 



