GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 65 



i. A short but measurable portion between the point of stimulation and 

 the first evidence of the shortening, a b, known as the " latent period." 

 The duration of this period for the skeletal muscle of the frog was 

 originally determined to be o.oi second, but with the employment 

 of more accurate apparatus it has been reduced to 0.002 5 to 0.004 second. 

 During this period it is supposed that certain chemic changes are 

 taking place preparatory to the exhibition of the movement. ^ The 

 duration of the latent period is influenced by a variety of conditions, 

 e.g., temperature, fatigue, strength of stimulus, etc. 



2 An ascending portion, b c, the contraction or period of increasing energy. 

 The contraction as shown by the character of the curve begins slowly, 

 then proceeds rapidly, and again slowly as the shortening reaches its 

 maximum. The contraction may be said to end when the tangent 

 to the curve becomes parallel with the abscissa. 



FIG. 24. THE ISOTONIC MYOGRAM. 



, A descending portion, c d, the relaxation or period of decreasing energy. 



The relaxation as shown by the character of the curve begins slowly, 



then proceeds rapidly, and again slowly as the muscle attains its 



original length. The termination of the relaxation is at the point where 



the curve cuts the abscissa. The curve beyond this point may be 



complicated by the presence of one or more residual or a ter-vibrations 



which are probably due to the inertia of the lever as well as to changes 



in the muscle elasticity. , , 



The duration of the period of shortening is about 0.04 second, an 



of the period of relaxation 0.05 second. A single pulsation o te isolated 



muscle of the frog therefore occupies, from the moment of stimulation ^to 



termination, the tenth of a second. Muscles of many other ammds have 



a contraction period the duration of which varies considerably from this. 



Thus in man the time of a single contraction is one-twentieth of a seco id, 



SSne in* Ss one three-hundredth of a second and in the turtle one second. 



fied by both external and internal conditions, among which may b 



proportional, within limits, to the strength of the latter Thu, using 

 as a stimulus the single induced current, it has been found that 



