58 HUMAN PHYSIOLOGY. 



the muscle curve (see Fig. 4), there is between the moment of stimu- 

 lation and the beginning of the contraction a short but measurable 

 period, known as the latent period, during which certain chemic 

 changes are taking place preparatory to the exhibition of the muscle 

 movement. Even when the electric stimulus is applied directly to 



FIG. 4. MUSCLE CURVE PRODUCED BY A SINGLE INDUCTION SHOCK APPLIED TO 



A MUSCLE. (Landois.) 

 a f. Abscissa, a c. Ordinate. a b. Period of latent stimulation, b d. Period 



of increasing energy, d e. Period of decreasing energy, e f. Elastic 



after-vibrations. 



the muscle, a latent period, though shorter, is observable. The 

 duration of this period in the skeletal muscles of the frog has been 

 estimated at o.oi of a second; but it has been shown by the em- 

 ployment of more accurate methods and the elimination of various 

 external influences to be much less not more than 0.0033 to 0.0025 

 of a second. 



The contraction follows the latent period. This begins slowly, 

 rapidly reaches its maximum, and ceases. This has been termed the 

 stage of rising or increasing energy. The time occupied in the stage 

 of shortening is about 0.04 of a second, though this will depend on 

 the strength of the stimulus, the load with which the muscle is 

 weighted, and the condition of the muscle irritability. 



The relaxation immediately follows the contraction. This takes 

 place at first slowly, after which the muscle rapidly returns to its 

 original length. This is the period of falling or decreasing energy, 

 and occupies about 0.05 of a second. The whole duration of a muscle 

 contraction occupies, therefore, about o.i of a second. 



Residual or after-vibrations are frequently seen which are due to 

 changes in the elasticity of the muscle. The amplitude of the con- 

 traction depends upon the condition of the muscle, the load, the 

 strength of stimulus, etc. 



Contraction of Non-striated Muscle. The curve obtained by 

 registration of the contraction of non-striated muscle shows that it 



