106 FEEDS AND FEEDING 



N 



the time he is not working, while when the tractor stops work, all ex- 

 penditure of energy ceases. 



144. The body not a heat machine. Sometimes the erroneous idea is 

 gained that the animal body is a heat machine ; i.e., that muscular work 

 is derived from heat produced by the oxidation of nutrients in the 

 muscular tissues. However, the body is not like a steam power plant, 

 in which the work is derived from the heat produced by the burning 

 coal. In the animal body the energy of the food is transformed into 

 work in quite another way, which is more similar to the production of 

 power in an internal combustion engine, such as the gasoline engine. In 

 this the part of the energy liberated by the explosion of the gas is con- 

 verted directly into the energy of motion and produces the work 

 accomplished. Another part is changed into heat and is wasted, so far 

 as any useful purpose is concerned, and must be gotten rid of by the 

 cooling system of the engine. Similarly, by some mysterious means a 

 sudden decomposition of the food nutrients occurs in the muscles of the 

 animal with the direct production of muscular work and also the 

 formation of heat as a waste product. 



A knowledge of the complicated processes going on in the animal body 

 in the regulation of its manifold activities causes us to marvel at its 

 economy and perfection. By processes still unknown the animal machine 

 produces muscular energy, heat, light, and electricity with surprising 

 efficiency. Moreover, with animals the fuel is burned at a low tem- 

 perature. The glow worm and firefly produce light without sensible loss 

 of heat or other energy and the torpedo fish and electric eel generate 

 electricity by means unknown. Scientists and inventors alike are baffled 

 by the mysterious and wonderful processes continuously occurring in the 

 animal body. 



As the horse is the principal animal machine for performing work, this 

 subject is appropriately continued in Chapter XVIII. 



II. PRODUCTION OF MILK 



145. Secretion of milk. Milk, the marvelous fluid designed by Nature 

 for the nourishment of the young of all mammals, is secreted by special 

 organs, called the mammary glands. Scientists disagree as to the exact 

 process by which the milk is formed in the small sac-like bodies, known 

 as alveoli, in the udder. However, we do know that the blood, laden 

 with nutrients, is brought by the capillaries of the udder to the alveoli. 

 The nutrients then pass thru the walls of the capillaries into the alveoli, 

 where by one of Nature's wonderful processes they are converted into 

 milk, which differs entirely in composition from the blood, whence it 

 originates. The chief proteins of milk casein and milk albumin differ 

 from all other proteins of the body, and the milk fat also has entirely 

 different properties from the body fat of the same animal. Milk sugar, 

 the carbohydrate of milk, is found nowhere else in the body. While the 



