78 TEXT-BOOK OF PHYSIOLOGY. 



ing hundreths of a second. The curve may be divided into three 

 portions : 



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 prepara- 

 tory to the exhibition of the movement. The duration of the 

 latent period is influenced by a variety of conditions, e. g., tem- 

 perature, 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 re- 

 laxation 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 irf 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 contrac- 

 tion is one-twentieth of a second, in some insects one three-hundreth 

 of a second, and in the turtle one second. Pale muscles have a shorter 

 period than the red. 



Influences Modifying the Contraction Process. The con- 

 traction process in its entirety as well as in its individual parts* is 

 considerably modified by external conditions, among which may be 

 mentioned the following: 



i. Stimulus. /As the contraction is the response of the muscle to 

 a stimulus, the vigor of the former is proportional, within limits, 

 to the strength of the latter. 1 ) Thus, with single induction shocks 

 the height of the contraction or the degree of shortening increases 

 as the strength of the stimulus increases from a minimum to a 



