642 A MANUAL OF PHYSIOLOGY. 



which is dim in ordinary light. There is no transference of it, and 

 no real reversal of the stripes. 



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 

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

 accordingly 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 myograph (Fig. 261, p. 707). 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 con- 

 siderably with temperature, fatigue, and other circumstances. It 

 is measured by the vibrations of a tuning-fork written immediately 

 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. The duration of the contraction of smooth 

 muscle evoked by a single momentary stimulus is much greater than 

 that of striped muscle (two to seven seconds for the rabbit's ureter ; 

 five to fifteen seconds for the cat's nictitating membrane ; one to 

 two minutes for the frog's stomach) . 



Latent Period. If the time of stimulation is marked on the 

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

 taneously with it, but only after a certain interval, which is called 

 the latent period. 



This can be measured by means of the pendulum myograph 

 (Fig. 229) or the spring myograph (Fig. 228), in both of which 

 the carrier of the recording plate opens, at a definite point in its 

 passage, a key in the primary coil of an induction machine, and so 

 causes a shock to be sent through the muscle or nerve, which is con- 

 nected with the secondary. The precise point at which the stimulus 

 is thrown in can be marked on the tracing by carefully bringing the 

 plate to the position in which the key is just opened, and allowing 

 the lever to trace here a vertical line (or, rather, an arc of a circle) . 

 The portion of the time-tracing between this line and a parallel line 

 drawn through the point at which the contraction begins gives the 

 latent period. 



Helmholtz measured the length of the latent period by means of 

 the principle of Pouillet, that the deflection of a magnet by a current 

 of given strength and of very short duration is proportional to the 

 time during which the current acts on the magnet. He arranged 

 that at the moment of stimulation of the muscle a current should 

 be sent through a galvanometer, and should be broken by the 



