402 PHYSIOLOGY 
as assimilation. To give it a name is about all that can be done at pres- 
ent, for until very much more is known of the chemistry of proteins, of 
which protoplasm chiefly consists, practically nothing can be known of 
the details of assimilation. 
Metabolism. The important steps in nutrition are these : (i) the 
making of carbohydrates in green parts properly lighted out of H 2 CO 3 ; 
(2) varied modification of these and incorporation of nitrogen (often also 
sulfur and phosphorus) from mineral salts to form amides and finally 
proteins; (3) the assimilation of proteins into protoplasm. On the whole 
these steps are upward; the material becomes, though with many 
fluctuations, gradually more and more complex, until it enters upon its 
final, most complex, least stable, living condition. It is maintained for 
a time at the high level as living stuff, or it becomes a part of some more 
permanent portion of the body, like the cell wall ; or it is broken up and 
reduced gradually to simpler compounds, some perhaps to be rebuilt 
into living matter again, some to break into simpler and simpler com- 
pounds and to leave the body (e.g. as CO 2 , H 2 , etc.). 
Metabolism is an old general name for all the chemical changes in 
a living organism. The constructive phases of nutrition are often 
summed up in the term anabolism or constructive metabolism; the de- 
structive phases as catabolism or destructive metabolism. In the former 
the processes tend to be synthetic; in the latter analytic. Having con- 
sidered the synthetic processes, the analytic ones demand attention in the 
next chapter. 
