OH. X.] THE SIMPLE MUSCLE CtJftVE. 123 



It will be observed that after the stimulus has been applied 

 as indicated by the vertical line s, there is an interval before the 

 contraction commences, as indicated by the line c. This interval, 

 termed (a) the latent period, when measured by the number of 

 vibrations of the tuning-fork between the lines s and c, is found to 

 be about y^sec. During the latent period there is no apparent 

 change in the muscle. 



The second part is the (6) stage of contraction proper. The 

 lever is raised by the contraction of the muscle. The contraction 

 is at first very rapid, but then progresses more slowly to its 

 maximum, indicated by the line mx, drawn through its highest 

 point. It occupies in the figure y^sec. (c) The next stage, 

 stage of elongation. After reaching its highest point, the lever 

 begins to descend, in consequence of the elongation of the muscle. 

 At first the fall is rapid, but then becomes more gradual vmtil 

 the lever reaches the abscissa or base line, and the muscle attains 

 its pre-contraction length, indicated in the figure by the line c. 

 The stage occupies ^^second. Very often after the main con- 

 traction the lever rises once or twice to a slight degree, producing 

 small curves (as in fig. 147). These contractions, due to the 

 elasticity of the muscle, constitute the (<l) Stage of elastic after- 

 vibration, or contraction remainder. 



The whole contraction occupies about T V of a second. With 

 regard to the latent period, it should be pointed out that if the 

 muscle is stimulated indirectly, i.e., through its nerve, some of the 

 apparent lost time is occupied in the propagation of the nervous 

 impulse along the nerve. To obtain the true latent period, this 

 must be deducted. Then there is generally latency in the 

 apparatus, friction of the lever, &c., to be taken into account. 

 This can be got rid of by photographing the contracting muscle, 

 on a sensitive photographic plate travelling at an accurately- 

 timed rate. By such means it is found that the true latent 

 period is much shorter than was formerly supposed. It is only 

 virr f a second. In red muscles it is longer. 



We now come to the action of various factors in modifying the character 

 of the simple muscle curve. 



1. Jnriiii /!<; i>f xtrrnijth nf stimulus. A minimal stimulus is that which 

 is just strong enough to give a contraction. If the strength of stimulus is 

 increased the amount of contraction as measured by the height of the curve 

 is increased, until a certain point is reached (maximal stimulus), beyond 

 which inc;-r:i-i- in the stimulus produces no increase in the amount of con- 

 traction. The latent period is shorter with a strong than with a weak 

 stimulus. 



2. Influrnre of load. Up to a certain point increase of load increases the 

 amount of contraction, boyond which it diminishes, until at last a weight 



