574 A MANUAL OF PHYSIOLOGY 



(by induced currents set up in a coil by the longitudinal vibrations of 

 a magnetized bar of iron), saw complete tetanus even at 24,000 

 stimuli a second ; while v. Kries in a cooled muscle found tetanus 

 replaced by the simple initial twitch at TOO stimuli per second, 

 although in a muscle at 38 C. stimulation of ten times this frequency 

 still caused tetanus. But it is doubtful whether the electrical method 

 of stimulation is capable of solving the problem, because of the 

 difficulty of being sure that the number of excitations is the same as 

 the nominal number of shocks, all the more that even very short 

 currents leave alterations of conductivity and excitability behind 

 them (Sewall), which we shall have to discuss in another chapter 

 (P- 59 6 )- 



It is only while the actual shortening is taking place that 

 a tetanized muscle can do external work. But although 

 during the maintenance of the contraction no work is done, 

 energy is nevertheless being expended, for the metabolism of 

 a muscle during tetanus is greater than during rest. Among 

 other changes, the carbon dioxide given off is increased, and 

 lactic acid produced. And upon the whole a muscle is 

 more quickly exhausted by tetanus than by successive single 

 contractions, although there are great differences between 

 different muscles. For example, the muscles which close the 

 forceps of the crayfish or lobster have, as everyone knows, 

 the power of most obstinate contraction. Richet tetanized 

 one for over seventy minutes, and another for an hour and 

 a half, before exhaustion came on, while a tetanus of a 

 single minute exhausted the muscles of the crayfish's tail. 

 The gastrocnemius of a summer frog kept up for twelve 

 minutes, and a tortoise muscle for forty minutes. 



Continuous stimulation is not always necessary for the production 

 of continuous contraction ; in some conditions a single stimulus is 

 sufficient. A blow with a hard instrument may cause a dying or ex- 

 hausted, and in thin persons even a fairly normal, muscle to pass into 

 long-continued contraction. This so called ' idio-muscular ' con- 

 traction seems to depend, in part at least, on the great intensity of 

 the stimulus. It can sometimes be obtained in the frog's gastroc- 

 nemius, particularly in spring after the winter fast. It is not a tetanus 

 and is not propagated along the muscular fibres, as an electrical 

 tetanus is, but remains localized at the spot where it arises. Similar 

 non tetanic contractions have already been mentioned, such as the 

 tonic contraction during the passage of a strong voltaic current and 

 the sustained veratria contraction. Ammonia causes also a long but 

 non-tetanic contraction, and this, too, does not spread when the sub- 



