11 I'lIVSIoLtMiV OF MIX'l.KS AND M.KVES. 



dinally ; for these al>o nnit a sound although n<> change 

 of form is externally percept ilile. 



This raises a qiie-tion as to how many of these irri- 

 tations are really requisite in order t<> bring a muscle 

 into ;in enduring condition of contraction. By means of 

 Warner's hammer (fig. 12), just described, or by means 

 of an electric wheel (fig. 11), the number of the irrita- 

 tions may be regulated. It will be found that from 16 to 

 indistinct irritations in each second are quite sufficient to 

 aii>e a constant emit raction of t he muscle. In a living 

 body also, where the muscles are voluntarily contracted, 

 the i -olid it ion of tetanus appears to be produced by the 



same number of irritations. It has been found that the 



height <if the muscle-note heard during voluntary eon- 

 tract ion of the muscles is about equal to c 1 or r/ 1 , which 

 ivpre-.'iit- from :>2 to 3f> vihrat ions in the second. Hut 

 llelmholtx was able to show, with great probability. 

 that this is not the t rue niimln r of mnscle-\ ibrat ions, 

 but that the vibrations \\ithin the muscle are really 

 only half as many. As, however, notes of this pitch 

 are indistinguishable to our ears, we hear the next 

 higher tone instead, which represents twice the num- 

 ber of \ ibrat ions. 1 



(i. As yet we have noticed only the shortening of 

 mii-cles. This alone determines 1 he amount of labour 

 accomplished, which consists in raising weights. But 

 on looking at a contracted muscle, it is evident that 

 it ha- beiome, not only <hort<T. but thicker. This 



1 Aci- Tiling t'> l'n-\rr, .-MIIII- ni' apable of distinguishing 



>f :is m:mv ;i< lit'irm tu t \vnit y-li vi- \ il.r:il ions per s.'CMUil ; 

 ,-iinl. ;icciriliiiL r In llic s:uiic .-nil Imrit v, tin 1 iiiii-,rlr-inil< > M.IIIH|S very 



ihiit I'l-i'ilui'i'il l-y lYinii -i.Lrliti i '-ii I" t\\i-nly vilr;il inns 

 second, \'. ry \\>-l\ \\ith (!< \.i-\\>ni' Helmholtz 



