198 



PHYSIOLOGY 



curve differs very little in form from the isotonic, displaying only a somewhat quicker 

 development of energy at the beginning of contraction. It is better to eliminate 

 the lever altogether and magnify the minute movements of the spring by attaching 

 to it a small hinged mirror by which a ray of light is reflected through a slit on to a 

 travelling photographic plate. Since the ray of light has no inertia, magnification 

 of the movements may be carried to any extent without increasing the instrumental 

 deformation of the curve (Fig. 56). 



A simple muscular contraction or twitch, such as that in Fig. 52, 

 produced by a momentary stimulus, consists of three main phases : 



(1) A phase during which no apparent change takes place in the muscle, 

 or at any rate none which gives rise to any movement of the lever. This 

 is called the latent period. 



FIG. 50. Myograph for optical registration of muscular 

 contraction. (K. LUCAS.) 



(2) A phase of shortening, or contraction. 



(3) A phase of relaxation, or return to the original length. 



The small curves seen after the main curve are due to elastic vibrations 

 of the lever, and do not indicate any changes occurring in the muscle itself. 

 From the time-marking below the tracing we see that the latent period 

 occupies about T - ( J () second, the phase of shortening T * (T , and the relaxation 

 yg-^ second. 



Thus a single muscle-twitch is completed in about r \ T second. It must 

 be remembered however that this number is only approximate, and varies 

 with the temperature of the muscle and its condition, being much longer in 

 a fatigued muscle. Moreover it is almost impossible to avoid some deforma- 

 tion of the curve due to defects of the recording instruments used. Thus the 

 relative period during which no mechanical changes are taking place in 

 the muscle must always be shorter than is apparent from a curve obtained 



