CH. X.] THE SIMPLE MUSCLE CURVE 117 



by the line mx, drawn through its highest point. It occupies in the 

 figure -o ^sec. 



The next stage is the 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 be- 

 comes more gradual until 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 o\nr sec - Very often after 

 the main contraction the lever rises once or twice to a slight extent, 

 producing small curves (as in fig. 147). These contractions are simply 

 due to the elasticity of the muscle and recording apparatus, and are 

 most marked when the contraction is rapid and vigorous. 



The whole contraction occupies about ^ 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 latency in the apparatus (friction of the lever, etc.) 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 Tt ^5- of 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. Influence of strength of 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 increase in the stimulus 

 produces no increase in the amount of contraction. The latent period is shorter 

 with a strong than with a weak stimulus. 



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

 amount of contraction, beyond which it diminishes, until at last a weight is reached 

 which the muscle is unable to lift. The latent period is somewhat longer with a 

 heavy load than with a light one. 



3. Influence of fatigue. This can be very well illustrated by letting the muscle 

 write a curve with every revolution of the cylinder, until it ceases to contract 

 at all. The next diagram shows the early stages of fatigue. At first the con- 



FIG. 140. Fatigue. 



tractions improve, each being a little higher than the preceding ; this is known as 

 the beneficial effect of contraction, and the graphic record is called a staircase. Then 

 the contractions get less and less. But what is most noticeable is that the contrac- 



