MECHANICAL PHENOMENA OF MUSCULAR CONTRACTION 719 



is dim in ordinary light. There is no transference of it, but, according 

 to most writers, the bands which are dim in ordinary light increase in 

 size by the transference of liquid from the isotropous band. 



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 con- 

 traction is to be recorded. The whole 

 arrangement for taking a muscle- 

 tracing is called a myograph (Fig. 

 278, p. 785). The duration of a 

 ' twitch ' or single contraction (in- 

 cluding 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 immediately 

 below or above the muscle curve. 

 When the muscle is only slightly 

 weighted, it but very gradually 

 reaches its original length after con- 

 traction, a period of rapid relaxation 

 being followed by a period of ' resi- 

 dual 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 con- 

 traction of smooth muscle evoked by a single momentary stimulus is 

 much greater than that of sti iped muscle (two to seven seconds for the 

 rabbit's ureter; five to fifteen seconds for the cat's nictitating mem- 

 brane; 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 simultaneously with fi, 

 but only after a certain interval, which is called the latent period. 



This can be measured by means of the spring myograph (Fig. 224) 

 or of the pendulum myograph, a pendulum which in its swing carries 

 a smoked plate against the writing-point of a lever connected with a 

 muscle. 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 



Fig. 243. Living Muscular Fibre (from 

 Gcotrupes stcrcorarius). i, in or- 

 dinary; 2, in polarized light. (Van 

 Gehuchten.) In living muscle (at 

 least in fibres which are not extended) 

 in contrast to dead muscle after treat- 

 ment with reagents, the doubly re- 

 fracting or anisotropous substance is 

 present in the greater part of the fibre ; 

 and with crossed nicols the position of 

 the singly refracting or isotropous 

 material is indicated only by narrow 

 transverse black lines or rows of dark 

 dots. 



