GENERAL PHYSIOLOGY OF MUSCLE-TISSUE 65 



1. 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 0.0025 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. 



3. 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 after-vibrations, 

 r 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, and 

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

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

 termination, the tenth of a second. Muscles of many other animals 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 second, 

 in some insects one three-hundredth of a second, and in the turtle one second. 

 Pale muscles have a shorter period than the red. 



Influences Modifying the Effect of the Stimulus. The contraction 

 process in its entirety as well as in its individual parts is considerably modi- 

 fied by both external and internal conditions, among which may be mentioned 

 the following: 



i. Character of the Stimulus. As the contraction is the response of the 

 muscle to a stimulus, it has been inferred that the vigor of the former is 

 proportional, within limits, to the strength of the latter. Thus using 



