ix NUTRITION 149 



now in a position to understand the general features of 

 the whole complicated series of processes which have to 

 do with the nutrition of the frog, which are collectively 

 spoken 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 substances which 

 ultimately take 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 substance, whereby the waste is made good, 

 and the tissues adequately nourished (constructive meta- 

 bolism). Oxygen is also withdrawn from the blood; 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 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 capillaries 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 



