EXHAUSTION AND EECOVERY. 79 



finally entirely ceases. The muscle is then said to be 

 exhausted. But if it is allowed to rest it recovers 

 itself so that it can again be induced to contract. This 

 recovery is, however, never complete, and with each 

 repetition of the experiment it becomes more defec- 

 tive, the intervals requisite for recovery becoming 

 continually longer, and the muscle finally remaining 

 incapable of further contraction. If the muscle is not 

 tetanised, but distinct pulsations are induced in it by 

 separate irritants, it retains its power of activity for a 

 very long time. From this it may be inferred that a 

 portion of the products of decomposition perhaps re- 

 form ; or it may be assumed that the muscle contains a 

 large amount of matter capable of disintegration, but 

 that this is capable of only gradual decomposition. So 

 long as the blood continues to flow through the muscle, 

 the products of decomposition are, as we have seen, 

 soon carried away ; but as exhaustion occurs in this case 

 also, we must draw the same conclusion, that the de- 

 composable matter present can undergo decomposition 

 only gradually, and that therefore in this case also 

 intervals must necessarily occur between the separate 

 exercises of activity. A muscle while undisturbed within 

 the organism essentially differs from one that has been 

 extracted in that in the former the expended material 

 can be fully replaced. Accordingly, it is not only capable 

 of again becoming active after an interval of rest, but, 

 provided that the matter added exceeds that which was 

 expended, it is afterward capable of performing more 

 work than it was previously. To this is due the fact 

 that the strength of muscle is increased by a proper 

 alternation of rest and activity. 



4. We have now to discover which of the substances 



