4 BULLETIN 459, U. S. DEPARTMENT OF AGRICULTURE. 
in water. Like the fats, they contain no nitrogen or sulphur, but 
they differ from the fats in containing less carbon and more oxygen. 
The carbohydrates of feeding stuffs may be divided into two © 
classes. The first of these includes those substances which are found 
in the cell walls and constitute the framework of the plant. This class 
includes cellulose and a great variety of other substances, most of 
which are rather difficult to dissolve. The “‘crude fiber” obtained in 
the analysis of feeding stuffs represents this class of carbohydrates. 
The second class of carbohydrates consists of the reserve material 
stored up in the cells and includes starch, the various kinds of sugar, 
and other less familiar substances. Some of these carbohydrates, like 
the sugars, dissolve in water and all may be converted into soluble 
forms rather easily. In analyses of feeding stuffs they are contained 
in the ‘‘nitrogen-free extract’”’ which, however, also includes a variety 
of other substances of ill-defined nature. 
THE ANIMAL BODY AS A MACHINE. 
Mechanically the body of an animal is a very wonderful machine, 
but what is of peculiar interest in this connection is that the body is 
what the engineer calls a prime motor—that is, like the steam or gaso- 
line engine, it moves itself and may supply power to move other 
machines. In fact, there is in some respects a very close likeness 
between the animal body and what are known as internal-combustion 
motors, i. e., those engines in which power is developed by burning 
liquid or gaseous fuel (gasoline, alcohol, producer gas, etc.) in the 
cylinder of the engine itself. Such an engine requires two things for 
its operation: (1) Sufficient repair material to keep its working parts 
in running order, and (2) a supply of fuel in proportion to the work 
to be done. Just these same two things are what the animal 
requires—repair material and fuel. 
In one respect, however, the animal body differs from the artificial 
machine—it can not be stopped and started again at will. As long as 
the animal lives the vital machinery is in operation, although less 
actively at some times than at others. The animal might be com- 
pared to an automobile whose engine must be kept running at a low 
speed in order to have the power available when needed. Conse- 
quently, the animal requires to be supplied with repair material and 
with fuel as long as it lives and not merely when it is in active use. 
That the feed of the animal is its source of both repair material and 
fuel is sufficiently obvious. We do not need a physiologist to tell us 
that when an animal is deprived of feed its tissues waste away while 
its fat is burned up in the effort to keep the bodily machinery in 
motion. We may proceed at once, therefore, to consider the feed in 
these two relations. 
