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 Richardson, 1897 ; Lucas, 

 1903, etc.) The principle is that the 



contracting muscle deflects a small 



I Te^V IL ^"X. mirror, from which a beam of light is 



reflected on to a travelling sensitive 

 surface so that the movement of con- 

 traction is photographed. 



The myograms best suited for 

 analysis and study are those ob- 

 tained from " nerve -muscle pre- 

 parations " of the frog or other 



Fio. 3. Frog's nerve -muscle preparation. Cold-blooded animal, ill which the 



muscle; n, sciatic exc it a bility of the nerves and 



t.a. 



nerve, with all the branches cut except 

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

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

 tenclo Achillis with hook to attach lower 

 end of muscle to myograph ; c.s/i., extreme 

 end of spinal cord. 



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 gastrocneinius by a break 



shock from the secondary coil of an induction apparatus. In 



order to determine the exact 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 



and others). 



In Fig. 4 three different 



periods can be distinguished: 



(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. 



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



PIG. 4. Myogram of contraction of frog's gastm- 

 cnemius. Time tracing from tuning-fork, giving 

 10U vibrations per second, n, li, latent period ; 

 li, <, phase of contract-ion ; <, d, phase of re- 

 laxation. 



