MANUAL OF CATTLE-FEEDING. 203 



for tlie flre^ driving the macliiiierj of a sliop, etc., so the 

 energy set free in tlie body takes various forms. It may 

 appear as heat, or as motion, it may take the form of elec- 

 trical currents, or it may produce chemical changes, such as 

 the formation of a complex compound out of simple ones. 

 All these phenomena we class together mider the general 

 name of the production of work. 



It is obvious at once that the production of work is an- 

 tagonistic to the formation of the material products for 

 which animals are frequently kept. All work is performed 

 at the expense of food or tissue, and the more work is per- 

 formed the less mateiial remains for the production of 

 flesh, fat, milk, etc. 



This is a common observation as regards external work 

 — no one would undertake to fatten a laboring animal — 

 but it is equally true of such less obvious forms of work 

 as the production of heat or of chemical change. Plainly, 

 then, we have a very practical interest in knowing what 

 constituents of the food or of the body are destroyed in 

 the performance of the various kinds of work, since all the 

 material losses thus occasioned nmst finally be supplied by 

 the food. 



One kind of work, viz., muscular exertion, has been the 

 subject of much study and controversy ; and though we do 

 not even yet know with certainty what substance or sub- 

 stances are the source of muscular power, yet what has 

 been learned is of great value. Other forms of work, on 

 the contrary, have received comparatively little attention, 

 and offer a wide field for investigation. 



In the following pages we shall take up first the pro- 

 duction of external work and its bearings on the feeding of 

 working animals, and then attend to a few considerations 

 concerning the internal work of the body. 



