8 



PHYSIOLOGY 



CHAP. 



the lever to progressively increasing weights suspended from the thread and 



wheel at the axis. 



The errors inseparable from the use of a lever (inertia, etc.) have more 



recently been eliminated by employing the photographic method (Blix, 1895 ; 



Brodie and Eichardson, 1897 ; Lucas, 

 1903, etc.) The principle is that the 

 contracting muscle deflects a small 

 mirror, from which a beam of light is 

 reflected on to a travelling sensitive 

 surface so that the movement of con- 

 traction is photographed. 



-/.* 



Fio. 3. Frog's nerve -muscle preparation. 

 m, gastrocnemius muscle ; n, sciatic 

 nerve, with all the branches cut except 

 that to the muscle ; /, femur ; p, clamp to 

 fix upper end of muscle with femur ; t.a. , 

 tendo Achillis with hook to attach lower 



The myograms best suited for 

 analysis and study are those ob- 

 tained from " nerve -muscle pre- 

 parations " of the frog or other 

 cold-blooded animal, in which the 

 excitability of the nerves and 

 muscles lasts much longer than 

 in warm-blooded animals (Fig. 3). 

 Whatever the nature of the 

 stimulus applied to the muscle or 

 its nerve, the contraction which is recorded by the myograph may 

 assume the form of a twitch or of tetanus. The twitch is the 

 simplest and most rapid form of muscular contraction ; tetanus is 

 a more complex and persistent contraction which results from the 

 fusion of a greater or less number of twitches in rapid succession. 



Fig. 4 gives the myogram of a simple twitch, obtained on the 

 momentary stimulation of the frog's gastrocnemius by a break 

 shock from the secondary coil of an induction apparatus. In 

 order to determine the exaqt moment at which the shock is 

 thrown into the muscle the 

 recording cylinder itself, at a 

 certain point of its revolution, 

 is arranged to open a contact 

 (Helmholtz), or else an electric 

 signal which is interposed in 

 the circuit marks the exact 

 moment of stimulation upon 

 the recording surface (Marey ^^^^^^^B^^ 



and Others). Fio- 4. Myogram of contraction of frog's gastro- 



T J 1.1. j-C cnemius. Time tracing from tuning-fork, gi\ in g 



in 1 Ig. 4 tnree dinerent 100 vibrations per second, a, b, latent period ; 



periods can be distinguished: JLatiolT 6 f contraction ; c> d ' phase of re " 



(a) The interval a b, in 



which no visible change takes place in the muscle; this is the 

 time lost between the application of the stimulus and the com- 

 mencement of the contraction, which Helmholtz termed the period 

 of latent excitation or latent period. 



(6) The interval b c, during which the muscle shortens, at first 



