4 BULLETIN 459, IT. 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. 



