480 LATENT PERIOD OF A MUSCLE CURVE. 



The muscle-curve proper is composed of 2, 3, and 4. 



1. The latent period (fig. 324, a, b) consists in this, that the muscle does not 

 begin to contract precisely at the moment the stimulus is applied to it, but the 

 contraction occurs somewhat later, i.e., a short, but measurable interval, elapses 



between the application of a 

 momentary stimulus and the 

 contraction (v. Helmholtz). If 

 the entire muscle be stimulated 

 by a momentary stimulus, e.g., 

 a single break induction shock, 

 the duration of the latent period 

 " is about '01 second. In smooth 

 Muscle-curve produced bya single induction shock ap- niuscle, the latent period may 

 plied to a muscle, a-/, abscissa ; a-c, ordinate ; a b, last for several seconds, 

 period of latent stimulation : b d, period of increasing [Although no change be visible 

 energy ; d e, period of decreasing energy ; e f, elastic in a musc l e during the latent 



period, nevertheless we have 

 proof that some change does take place within the muscle-substance, for we know 

 that the electrical current of the muscle is diminished during this period, or we 

 have what is known as the negative variation of the muscle-current {Bernstein 

 333).] 



In man, the latent period varies between - 004 and 0*01 second. If the experiment be so 

 arranged that the muscle can contract as soon as the stimulus is applied to it, i.e., before time 

 is lost in making the muscle tense ; or to put it otherwise, if the muscle has not " to take in 

 slack," as it were, the lateut period may fall below 0*004 second {Gad). If the muscle be still 

 attached to the body, protected as much as possible from external influences and properly 

 supplied with blood, the latent period may be reduced to 0*0033 or even 0'0025. 



Modifying Influences. The latent period is shortened by an increased strength of the 

 stimulus and by heat ; while fatigue, cooling, and increasing weight lengthen it (Lauterbach, 

 Mendelssohn, Yeo, Cash). The latent period of a break-contraction is longer than that of 

 a make-contraction. The red muscles have a longer latent period than the white. Before the 

 muscle contracts as a whole, the individual fibres within it must have contracted. We must, 

 therefore, conclude that the latency of the individual muscular elements is shorter than that of 

 the entire muscle {Gad, Tigerstedt). 



2. The contraction or stage of increasing energy, i.e., from the moment the 

 muscle begins to shorten until it reaches its greatest degree of contraction (b d). 

 At first the muscle contracts slowly, then more rapidly, and again more slowly, so 

 that the ascending limb of the curve has somewhat the form of an/. This stage 

 lasts 0*03 to 0*4 second. It is shorter when the contraction is shorter (weak 

 stimulus) and the less the weight the muscle has to lift. It also varies with the 

 excitability of the muscle, being shorter in a fresh, non-fatigued muscle. 



3. Elongation or stage of decreasing energy. After the muscle has con- 

 tracted up to its maximum for any particular stimulus, it begins to relax at first 

 slowly, then rapidly and lastly more slowly, so that an inverse of an /is obtained 

 (d e). This stage is usually of shorter duration than 2. The duration varies with 

 the strength of the stimulus, being shorter than 2 with a weak stimulus, and 

 longer with a strong stimulus. It also depends upon the extent to which the 

 muscle is loaded during contraction. 



^4. The fourth stage has received various names stage of elastic after-vibra- 

 tion [residual contraction or contraction-remainder (Hermann). The after- 

 vibrations (e /), which disappear gradually, 'depend upon the elasticity of the 

 muscle. The duration of this stage is longest with a powerful contraction, and 

 when the weight attached to the muscle is small]. 



If the stimulus be applied to the motor nerve instead of to the muscle itself, the 

 contraction is greater (Pftuger), and lasts longer (Wundt) the nearer to the spinal 

 cord the stimulus is applied to the nerve. 



