Production of Heat and Work. 85 



such was not the case. Measured by the nitrogen in the urine, the 

 protein broken down during the trial could not possibly have fur- 

 nished energy for more than one-third of the work done by these 

 men in lifting their bodies to the top of the mountain. -. 



From this trial and experiments by Voit, Pettenkof 3r, and Parks, 

 it was decided that only carbohydrates and fats were oxidized and 

 burned in the production of muscular energy. Still later experi- 

 ments by Argutinsky, Zuntz, and others have shown that when 

 carbohydrates and fat are sufficient in amount they furnish all the 

 muscular energy, and in such cases the breaking down of protein is 

 not increased during work. However, if the supply of carbohy- 

 drates and fat in the food is insufficient, some of the energy for the 

 production of work may be furnished thru the breaking down of 

 protein, with a resultant increase in the nitrogen excretion in the 

 urine. 



107. Excretion of carbon dioxid. Whether the material burned 

 to furnish muscular energy be carbohydrates, fat, or the non-nitrog- 

 enous part of the protein molecule, carbonic acid gas will be pro- 

 duced, the quantity directly depending upon the amount of work 

 done. This was shown by Smith, 1 who determined the quantity of 

 carbonic acid gas exhaled by the horse when at rest and perform- 

 ing labor as follows : 



Cubic feet 

 Form of work per hour 



At rest 1.03 



Walking 1.10 



Trotting 2.95 



Cantering 4.92 



Galloping 14.97 



Thus, unlike the nitrogen excretion, the amount of carbon dioxid 

 exhaled per hour is increased by the performance of work, and de- 

 pends upon the work done in that time. 



108. 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 yet 

 know definitely the direct cause of muscular contraction. In just 

 what manner the energy stored in the food is converted into the 

 energy of muscular action is still an unsolved question. We do 

 know, however, some of the processes which take place in the work- 

 ing muscles. 



The most significant change which takes place during muscular 

 contraction is the increased production of carbon dioxid already 

 1 Jour. Physiol., 1890, No. 1 ; U. S. Dept. Agr., Office of Expt. Sta., Bui. 22. 



