102 FEEDS AND FEEDING 



Frora this trial:- and experiments by Voit, Pettenkoffer, and Parks, it 

 was believed for a time that the carbohydrates and fats served as the only 

 source of muscular energy. The whole matter was cleared up only 

 after still later investigations by Kellner, Argutinsky, and Zuntz. These 

 trials showed that when the food contains sufficient carbohydrates and fat, 

 these nutrients furnish all the energy used up in work, and in such cases 

 the breaking down of protein in the body is not increased. On the other 

 hand, if the supply of carbohydrates and fat in the food is insufficient, 

 some of the energy for the production of the work will be furnished thru 

 the breaking down of protein, with a resultant increase in the nitrogen 

 excreted in the urine. Because the feed supplied work animals is usually 

 composed chiefly of carbohydrates, this class of nutrients ordinarily 

 furnishes practically all the energy animals use in their labor. 



Investigations by Zuntz and his associates 2 show that the value of each 

 of the different classes of food nutrients for the production of work de- 

 pends on the amount of net energy which each class furnishes the 

 animal. This is because the net energy, or the amount remaining after 

 deducting all the losses which occur in the feces, urine, and com- 

 bustible gases, and in the "work of digestion," is the only part of 

 the total energy of the feed which can be turned into muscular work. 



140. Production of muscular energy. We know that in doing work 

 the muscles of the body contract; that is, become shorter and thicker. 

 Yet, in spite of all the study of scientists, we do not know definitely 

 the direct cause of muscular contraction. How the nutrients stored in 

 the muscular tissues are converted into the energy of muscular action 

 is still an unsolved question. We do know, however, some of the proc- 

 esses which take place in the working muscles. 



The most significant change which takes place during muscular con- 

 traction is an immediate and marked increase in the production of carbon 

 dioxid in the body. By determining the amount of this gas exhaled from 

 the lungs, it has been found repeatedly that, unlike the amount of 

 nitrogen voided, the amount of carbon dioxid which is excreted depends 

 directly on the amount of work performed. For example, Smith 3 found 

 that when at rest a horse exhaled 1.03 cubic ft. of carbon dioxid per hour. 

 When the horse was walking slowly the excretion rose to 1.10 cubic ft., 

 and as the gait became faster and consequently the rate of work in- 

 creased, the amount became much larger, as follows: When trotting, 

 2.95 cubic ft. per hour ; when cantering, 4.92 cubic ft. ; and when gallop- 

 ing 14.97 cubic ft. 



During work there is also a large increase in the amount of oxygen 

 taken up by the muscles from the blood and an increased production of 

 water in the muscular tissues. The increase in oxygen consumed and 

 carbon dioxid given off might lead to the conclusion that the activity of 

 the muscle during contraction is du<j to simple oxidation, such as occurs 

 when fuel is burned. However, certain facts which cannot be dwelt 

 "Arch. Physiol. (Pfliiger), 83, 1901, p. 564. 3 Jour. Physiol., 1890, No. 1. 



