62 TEXT-BOOK OF PHYSIOLOGY. 



With the muscle arranged as previously described and stimulated 

 directly with a single induced electric current, the contraction will be re- 

 corded in the form of a curve similar to that represented in Fig. 24, in which 

 the horizontal line represents the abscissa of time; a, the moment of stimula- 

 tion; and bed, the degree of shortening and the subsequent relaxation at each 

 successive moment. The undulating line shows the time relations, the 

 distance from crest to crest representing hundredths of a second. The 

 curve may be divided into three portions : 



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 



FIG. 24. THE ISOTONIC MYOGRAM. 



taking place preparatory to the exhibition of the movement. The 

 duration of the latent period in 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. 



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, 

 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, 



