RATE OF THE MECHANICAL RESPONSE. 383 



the minimum delay which intervenes between instantaneous excitation of the 

 nerve and the first observable change of form is about 7-thousandths of a 

 second, and that in direct excitation this is diminished to about 3 i -thousandths. 

 The Avell-known observations of Bernstein x on the period of latency, which 

 were published in 1882, and made by the ordinary myographic method, gave 

 of course a much longer delay in both cases ; but the difference in this respect 

 between direct and indirect stimulation was as nearly as possible the same, 

 namely, 0*0032 sec. The inference drawn from the observation of this 

 difference was that in indirect stimulation about ^^ sec. is lost in transmission 

 through the end-organs of the nerve. Doubt was thrown on the significance 

 of Bernstein's experimental data by Hoisholt 2 in 1885, who concluded that 

 inasmuch as when an induction current is led through a muscle the muscle 

 fibres are excited partly by its direct action on them, partly by its action on 

 intramuscular nerves, the greater promptitude of the response might be 

 attributed to the summation of the two actions. The question has lately been 

 reinvestigated by Boruttau, 3 who has shown that, provided that strong (" over- 

 maximal ") currents are used, the abbreviation of the delay in direct stimula- 

 tion is always in accordance with Bernstein's statement ; he found, however, 

 that when the current strength is slightly diminished, the period of latency 

 acquires the duration which it possesses when the muscle is stimulated 

 through its nerve. 



•^' 



Rate of propagation of the mechanical response. — When a 

 curarised parallel-fibred muscle is subjected to the direct action of an 

 instantaneous stimulus at any spot in its length, in such a way that all 

 the fibres are affected by it, a thickening occurs at the excited spot ; 

 this at once divides into two parts, which separate from one another, 

 and are propagated in opposite directions. Each of these advancing 

 thickenings is called a contraction wave. In ordinary skeletal muscle of 

 the frog, it travels, provided that the preparation is fresh and vigorous, 

 at a rate which approaches 3 metres per second, and persists at each 

 point in its course for about ^ of a second, so that if the muscle were 

 sufficiently long, it would be observed that at the moment the thickening 

 ceases at one point it begins about 30 cms. further on in its course. 

 In other words, the contraction wave has a length of about T \ of a 

 metre. 



The contraction wave is a phenomenon intimately connected with 

 two others ; with the excitation wave on the one hand, and with the 

 duration of the twitch, or response to instantaneous stimulation, on the 

 other. By the term excitation wave is meant the initial change in 

 the excitable mechanism contained in every contractile element which, 

 in its progress, precedes the change of form, and is the harbinger of its 

 advance. We shall learn subsequently how it can be observed and 

 its rate of propagation determined. The duration of the twitch is the 

 time which intervenes between the moment at which any particular 

 part of a fibre or bundle of fibres departs from, and that at which it 

 returns to, its normal form. It can therefore be better determined by 

 measuring the change in thickness than the change in length of the 

 muscle. 



The first measurements of the rate of progress of the contraction 



1 "Die Erregungszeit der Nervenendorgane in den Muskeln," Arch. /'. Physiol., Leipzig, 

 1882, S. 329. 



2 Journ. Physiol., Cambridge and London, vol. vi. p. 1. 



3 " Zur Frage der spee. Erregungszeit der motor. Nervenendigungen," Arch. f. 

 Physiol., Leipzig, 1892, S. 454. 



