METABOLISM. 17 
various processes of nutrition, ingredients of the food are first incor- 
porated into the tissues of the body, to be subsequently broken 
down and oxidized. In this building-up process changes undoubt- 
edly occur in the direction of greater complexity of molecular struc- 
ture, involving the temporary absorption of energy. Thus it is 
known that fats may be formed from carbohydrates in the body. 
Many physiologists hold that the metabolism in the quiescent muscle 
results in the building up of a complex “contractile substance,” 
whose breaking down furnishes the energy for muscular work. In 
general, we may regard it as highly probable that the molecules of 
the living substance of the body are much more complex than those 
of the nutrients of the food, and that the former are built up out of 
the latter by synthetic processes, carried on at the expense of energy 
derived from the breaking down of other molecules. Such changes 
as this are called anabolic and the process anabolism, while the 
changes in the direction of greater simplicity of molecular structure 
are called katabolic, and the process katabolism. The metabolism 
of the living body, then, consists of both anabolism and katabolism. 
By the former the food nutrients are built up into body material; 
by the latter they are broken down, yielding finally the compara- 
tively simple excretory products. On the whole, however, the 
katabolism prevails over the anabolism, so that metabolism as a 
whole is, as already stated, an analytic and oxidative process. 
Neither the anabolism of tissue production nor the minor anabolic 
changes which seem to occur in various tissues alter the main direc- 
tion of the metabolic changes in the body, but, from the standpoint 
of the statistics of nutrition, are simply eddies in the main current 
§1. Carbohydrate Metabolism. : 
HEXOSE CARBOHYDRATES. 
The hexose carbohydrates of the food appear to be resorbed 
chiefly by the capillary blood-vessels of the intestines. For the 
most part, they reach the blood in the form of dextrose, with smaller 
amounts of levulose and with greater or less quantities of acetic, 
butyric, lactic, and other acids derived chiefly from the fermenta- 
tion of the carbohydrates in the digestive tract. In the general 
circulation only dextrose is found. 
