THE SOURCE OF MUSCULAR POWER. 821 



firmed by Henneberg's experiments on oxen. More carbonic acid is 

 excreted by day than by night, since more work is then done. But at 

 the same time less oxygen is taken into the body in the daytime than 

 during the night. For example, in one of Voit and Pettenkofer's ex- 

 periments, for every 100 parts of oxygen which entered the system in 

 the daytime 175 parts were contained in the carbonic acid excreted, 

 while in the night the same relation was expressed by the number 58. 

 When work was performed the difference was still greater. This and 

 similar experiments show plainly that a large part of the carbonic acid 

 excreted is formed at the cost of oxygen previously laid up in reserve, 

 and that the increased rapidity of respiration during work is not for the 

 purpose of supplying more oxygen, but of removing the carbonic acid. 



It has been also shown that the amount of oxygen that can thus 

 be stored up in health is proportioned to the amount of albuminoids 

 in the food, and this is another indication of the importance of these 

 bodies in the production of muscular power. 



The necessity of this storing up of oxygen is strikingly shown by 

 experiments on two diseases in which the patient is almost incapable 

 of muscular exertion, viz., diabetes mellitus and leukaemia lienalis. 

 In these diseases the total excretion and the total amount of food are 

 not much different from those in health ; but there is no such storing 

 up of oxygen as in the healthy organism, and there is also an almost 

 entire lack of strength. 



This fourth condition is, for our present purpose, the most interest- 

 ing and important of all. ' It shows 'that work is not produced by the 

 direct oxidation of food materials by the oxygen of the blood, but that 

 the muscles themselves contain a store of latent energy which the will 

 can set free at pleasure, independently of oxygen, while the blood serves 

 to wash out the waste products and gradually to renew the supply of 

 force during those periods of rest of which this fact explains the ne- 

 cessity. 



That the seat of this latent energy is in the muscles is shown by 

 the fact that they are capable of contraction for a time after tHeir 

 blood-supply has been cut off, or even after their removal from the 

 body. A frog's heart, when removed from the body and freed from all 

 blood by injection of a weak solution of salt, will continue to beat for 

 hours, and the whole animal under the same ch-cumstances moves, 

 leaps, and behaves in short like a living animal. Agassiz relates that 

 on one occasion he captured a shark which fought as long and fiercely 

 as is usual with these animals, but which, when finally secured, was 

 found to have its gills eaten through by parasites, and almost all its 

 blood replaced by sea- water. (Liebig.) 



Iiike a bent spring the muscle contains a certain amount of poten- 

 tial energy, which the will can use at pleasure ; but when the supply is 

 once exhausted, when the spring has lost its tension, a further supply 

 of force from without is necessary before more work can be performed. 



