CHAPTER VI. 



NUTRITION. 



HAVING seen how the food is digested, we next naturally inquire as to 

 the process by which the protoplasm of the organism is supplied with its 

 nutritive material, and how and for what purposes this organized fuel 

 is used by the body. 



By nutrition we mean a complex of vital processes in part included 

 under the term metabolism, but comprising also the two procedures, 

 absorption and excretion. In this chapter then we shall take up the 

 ultimate food-materials as they are left by the digestive juices of the gut 



FIG. Ill 



i^p^fl 



* 



w // 



^S) 



a 



The summit of the villus in the gut of a, kitten, showing the absorption of fat by the 

 various sorts of cells. (Heidenhain.) 



and leave them only when they have been crudely traced through their 

 changes in the body and excreted outside the organism again. These 

 processes of nutrition in part form a link of this physiological chain- 

 namely, that link which connects the digested food with the blood and 

 tissues it supplies. So far as metabolism proper is concerned there are 

 two sorts of processes, like two sides of an isosceles triangle (see Fig. 8), 

 the apex of which between them is the normal composition of the blood 

 and tissues. The up-going side is anabolism, constructive assimilation; 

 the down-going side represents the katabolism, destructive dissimilation. 

 Introductory to these two (metabolism) is absorption, while as a neces- 

 sary consequence of them excretion must be considered. The former 

 process introduces the nutriment actually into the blood and tissues; 



