102 THE HUMAN BODY 



fatigue, and its study is exceedingly instructive in aiding us to 

 analyze the workings of the muscle machine, but it is not the type 

 of fatigue that human beings most commonly feel. Our muscles 

 are, to be sure, so much like the muscles of the frog that they 

 would, if isolated and stimulated, show similar fatigue, but as 

 used in the body they are usually saved from experiencing serious 

 fatigue by the fact that the swiftly flowing blood tends to sweep 

 out the " fatigue products" and prevent their accumulation, and 

 also because in the nervous system, by which the muscles are 

 stimulated to activity, there are regions which become fatigued 

 while the muscles are yet in good condition. Only under excep- 

 tional circumstances, as in athletic contests, or severe manual 

 labor, when the nervous system is driven far beyond its usual ac- 

 tivity and the blood is unable to remove the waste products as 

 fast as they are formed, do we experience genuine muscular fatigue. 



The Response to Rapidly Repeated Stimuli. Tetanus. Since 

 a simple muscular contraction occupies one-tenth of a second, it is 

 obvious that a second stimulus following the first within that in- 

 terval will find the muscle in a state, of partial or complete contrac- 

 tion, depending on the exact length of the interval. In such a case 

 the muscle executes in response to the second stimulus a second 

 contraction, which is fused with the first. Similarly a third can 

 be elicited, which will be fused with the second, and so on. A 

 series of such fused contractions is known as a physiological tetanus. 

 If the interval between stimuli does not exceed yV-yV f a second 

 in fresh frog's muscle the fusion is complete. That is, there are 

 no signs of relaxation between stimuli, and the contraction is per- 

 fectly steady; An interesting fact about physiological tetanus, or 

 tetanic contraction as it is often called, is that the extent of the con- 

 traction may be markedly greater than in a simple contraction, 

 even though the latter may have been maximal. Evidently the 

 so-called maximal simple contraction is not maximal in the sense 

 that it represents the mechanical limit of the muscle's power to 

 shorten, but only in that it is the utmost the muscle can do in 

 response to a single stimulus. In two respects, then, the tetanic 

 contraction differs from the simple twitch: in being more pro- 

 longed, and in being higher. 



Voluntary Muscular Contraction. In view of the superiority 

 of the tetanus over the simple twitch it is interesting to note that 





