CH. IX.] THE SIMPLE MUSCLE CURVE 97 



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 4-^ 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 produce 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. Increase of load decreases the amount of 

 contraction, 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. Fig. 119 shows the 

 result. At first the contractions 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 curves are much more prolonged ; the latent period gets longer ; 

 the period of contraction gets longer ; and the period of relaxation 

 gets very much longer ; this condition is known as contracture, so 

 that the original base-line is not reached by the time the next 

 stimulus arrives. In the last stages of fatigue, contracture passes 

 off. Contracture is often absent in fatigue of mammalian muscle. 



4. Effect of temperature. Cold at first increases the height of 

 contraction, then diminishes it ; otherwise the effect is very like that 

 of fatigue, increasing the duration of all stages of the curve. 



Moderate warmth increases the height and diminishes the 

 duration of all stages of the curve, latent period included. This may 

 be readily shown by dropping salt solution * at different tempera- 



* Physiological saline solution used for bathing living tissue is a 0'9 per cent, 

 solution of sodium chloride in ordinary tap water. 



G 



