flOW Tflfi ANIMAL LIVES 229 



6. Work; Waste; Eest 

 6a. Waste of tissue 



422. Under bodily labor, the elements of the 

 muscles are used up to a certain extent, while 

 heat and waste matters are produced. A 

 period of rest is required to allow for repair of 

 this waste. We see this carried out in all healthy 

 bodily functions. The heart, after each contrac- 

 tion, has a short rest before the commencement 

 of the next contraction. The muscles that carry 

 on breathing work in relays, those that dilate the 

 chest resting while those that compress the 

 chest are in operation. Then both rest for an 

 interval before the next inspiration is com- 

 menced. This provides for rest and repair of 

 both the muscles and nerves. Except for such 

 rest, both would soon be exhausted and wasted 

 beyond the power of work. 



423. The waste of tissues, however, is not 

 always in exact proportion to the amount of 

 work. On the contrary, it has been shown by 

 careful experiment that the waste of the working 

 muscle is but a small part of the expenditure 

 made. The heat- or fat-producing matters in 

 the food are also used up in such work. 

 The process may be likened to fuel supplied to 

 the engine, which contributes to keep it running 



