86 Feeds and Feeding. 



noted, which seems to bear a definite relation to the amount of in- 

 ternal and external work performed. There is also a large increase 

 in the amount of oxygen taken up by the muscles from the blood 

 during work. The increase in oxygen consumed and carbon dioxid 

 given off might lead to the conclusion that the activity of the mus- 

 cle during contraction is due to simple oxidation, such as occurs 

 when fuel is burned. Certain facts which cannot be dwelt upon 

 here lead scientists, however, to believe that the chemical changes by 

 which energy is liberated are not simple oxidations, but are more in 

 the nature of sudden decompositions or cleavages of some complex 

 substance or substances built up in the muscle during rest, carbon 

 dioxid being evolved in such cleavage.^ Part of the energy liber- 

 ated in this decomposition appears as heat, and another part as me- 

 chanical work. 



Glycogen, or animal starch, is stored in the muscle during rest, 

 forming between 0.5 and 0.9 per ct. of the weight of well-nourished 

 muscle in the resting condition. (52) A smaller quantity of glu- 

 cose is also found in the muscular tissues. During muscular activ- 

 ity, this stored glycogen and glucose disappear more or less in pro- 

 portion to the extent and duration of the contractions, so that after 

 prolonged muscular activity or hard work the supply may be en- 

 tirely exhausted. Tho the amount of these carbohydrates in the 

 body tissues at any one time is small, the supply, especially of glu- 

 cose, is being continuously produced from the food nutrients or 

 body tissues to replace that oxidized in the production of work. As 

 the larger part of the food of farm animals consists of carbohy- 

 drates, the oxidation of the glucose formed from them probably fur- 

 nishes most of the energy for the production of heat and work by 

 these animals. 



To supply the muscles with the necessary oxygen and also carry 

 away the waste products formed during muscular exertion, the cir- 

 culation of the blood must be hastened and larger quantities of air 

 be taken in by the lungs. 



109. Source of muscular energy.— All the organic nutrients ab- 

 sorbed from the food, not only the carbohydrates and fats, but also 

 the proteins and apparently the pentosans, serve as the source of en- 

 ergy to the body. Under normal conditions the non-nitrogenous 

 nutrients and the glycogen are first drawn upon for the produc- 

 tion of work, no more protein being broken down than during mus- 

 cular rest. If the non-nitrogenous nutrients do not suffice for the 



^ Armsby, Principles of Animal Nutrition, 1903, p. 187. 



