292 DOMESTIC ANIMALS, DAIRYING, ETC. 



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 gasoline 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 arti- 

 ficial 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 food 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, but at the same time must weigh the effects of 

 all kinds of food as regards their comparative values in the economy 

 of fat-production, heat-generation and muscle formation. It is not 

 so much what the animal needs nor what the feed will furnish in the 

 way of heat, energy and strength, so much as it is in what the animal 

 can transform into those effects from the feed. In other words, the 

 desired elements in the feed must be so available that the particular 

 animal can transform them into the power required. (This leads 

 to a study of both animal digestion and plant nutrition.) 



