74 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



bringing of nutriment and oxygen and the removal of waste matters under 

 ordinary conditions. 



Considerable difference of opinion exists as to which of three classes of 

 food-stuffs — proteids, carbohydrates, and fats — supply the energy used by the 

 muscle in ordinary and excessive work, and how these are employed by the 

 muscle. 



The question has been studied by examining the character and quantity 

 of waste products liberated from the body during and after excessive mus- 

 cular work, as compared with those given off" when the subject is at rest. 

 Another method has been to test the strength of the muscle in ergographic 

 experiments, and to find the effect of different kinds of food upon the time 

 required for its recovery. Experiments of Kick and Wislicenus, 1 Voit and 

 Pettenkofer, 2 Voit, 3 and others caused the view to become generally accepted 

 that the energy of the muscle by violent muscular work comes largely from 

 the non-proteid substances in the muscles. Later Pfluger and his pupils 

 have gone to the other extreme and conclude that proteid is the chief source 

 of energy. 1 



Very many others have written on both sides of the subject and still a 

 final conclusion has not been reached. 5 



Probably the sugars, and possibly after these the fats are employed by the 

 muscle as the most available form of energy, while the proteid forms a more 

 permanent part of the muscular machine, and is only made use of when the 

 work is exhaustive (see page 166). The taking of any one of these classes 

 of food hastens the recovery from fatigue, and the sooner the more readily it 

 is digested and assimilated (see Metabolism — effect of muscular work). 



Normally the muscles are never completely fatigued. It would seem 

 that as the muscles tire and their irritability is lessened, the central nerve- 

 cells which send the stimulating impulses to them have to work harder, 

 and that the nerve-cells give out sooner than the muscles. On the other 

 hand, certain experiments seem to show that the nerve-cells recover from 

 fatigue more rapidly than the muscles do, so that it is an advantage to 

 the organism that they should cease to excite the muscles before muscular 

 fatigue is complete. With the decreasing irritability of the muscle, a feeling 

 of discomfort in the muscle and an increasing sense of effort are experienced 

 by the individual, both of which tend to cause a cessation of contraction, and 

 prevent a harmful amount of work. That such an arrangement would be of 

 service was apparent in the experiments of Maggiora, in which he found that 

 if muscles are forced to work after fatigue has developed, the time of recovery 

 is prolonged out of all proportion to the extra work accomplished. 



At the close of even exhaustive muscular work there is always a large 

 amount of energy-holding materials in the blood and tissues, and the rapid, 



1 Vierteljahrexschrift der naturforsche Gesclkchafl in Zurich, 1865, Bd. X. S. 317. 



2 Zeilschnfl fur Biologic, 1866, Bd. ii. 3 Ibid., 1876, Bd. vi. S. 305. 

 1 Pfluger' s Arclav, 1899, Bd. 77, S. 425. 



s Schafer's Text-book of Physiology, 1898, vol. i. p. 912. 



