MUSCLE 559 



tractive Dobie's globule are there approximated. The stripe which 

 in the contracted fibre is the brighter of the two in ordinary 

 light is the dimmer of the two in the field of the crossed nicols, 

 although it is not absolutely dark, since the shafts of the Bowman's 

 elements cause some rotation of the plane of polarization even in the 

 absence of the stainable material (Rutherford). 



Diffraction Spectrum of Muscle. When a beam of white light 

 passes 'through a striped muscle, it is broken up into its constituent 

 colours, and a series of diffraction spectra are produced, just as 

 happens when the light passes through a diffraction grating (a piece 

 of glass on which are ruled a number of fine parallel equidistant 

 lines). The nearer the lines are to each other, the greater is the dis- 

 placement of a ray of light of any given wave-length. It has accord- 

 ingly been found that when a muscular fibre contracts, the amount 

 of displacement of the diffraction spectra increases. At the same 

 time the whole fibre becomes more transparent. 



(2) Mechanical Phenomena. The muscular contraction may 

 be graphically recorded by connecting a muscle with a lever 

 which is moved either by its shortening or by its thickening. 

 The lever writes on a blackened surface, which must travel 

 at a uniform rate if the form and time-relations of the 

 muscle-curve are to be studied, but may be at rest if only 

 the height of the contraction is to be recorded. The whole 

 arrangement for taking a muscle-tracing is called a myograpb 

 (Fig. 192). The duration of a ' twitch ' or single contraction 

 (including the relaxation) of a frog's muscle is usually given 

 as about one-tenth of a second, but it may vary considerably 

 with temperature, fatigue, and other circumstances. It is 

 measured by the vibrations of a tuning-fork written imme- 

 diately below or above the muscle curve. When the muscle 

 is only slightly weighted, it but very gradually reaches its 

 original length after contraction, a period of rapid relaxation 

 being followed by a period of ' residual contraction,' during 

 which the descent of the lever towards the base line becomes 

 slower and slower, or stops altogether some distance 

 above it. 



Latent Period. If the time of stimulation is marked on 

 the tracing, it is found that the contraction does not begin 

 simultaneously with it, but only after a certain interval, 

 which is called the latent period. 



This can be measured by means of the pendulum myo- 

 graph or the spring myograph, in both of which the carrier 



