426 MOVEMENTS 



When the nerve is stimulated by means of a single feeble inductive 

 shock, the muscle contracts almost immediately after the electric dis- 

 charge. There is, however, a short interval between the stimulation 

 and the contraction. This is called the latent period, and it occupies 

 about I^-Q of a second. Then follows the stage of muscular contraction, 

 which rapidly progresses to its maximum. The period of contraction 

 occupies about ^ of a second. The muscle then rather more slowly 

 returns to its relaxed condition. This period occupies a little less than 

 T X 2 of a second. Not counting the latent period, the duration of a mus- 

 cular contraction and relaxation is about ^ of a second. A part of the 

 latent period is occupied in the conduction of the stimulus along the 

 nerve, and the part, belonging to the muscle is much shorter. Fol- 

 lowing the relaxation, there is often a vibratory wave of very slight 

 contractions, due to the elastic reaction of the muscular fibres. 



A stimulus may be applied to a nerve, that is of just sufficient strength 

 to produce a muscular contraction. This is called the minimal strength 

 of stimulus. By gradually increasing the strength of the electric cur- 

 rent, a maximal stimulus may be reached. The vigor of the muscular 

 contraction is proportional to the strength of the stimulus, between these 

 two extremes. The latent period is shorter with a strong than with a 

 weak stimulation. The vigor of contraction is not increased by increas- 

 ing the strength of the stimulus beyond the maximal point. 



The foot in a muscle-nerve preparation may be loaded with a weight. 

 When this is done, provided the weight be not too great to be lifted by 

 the muscle, the extent of the contraction increases with the weight up 

 to what may be called the maximum of contraction. Beyond this point, 

 however, it diminishes until the weight is increased beyond the power 

 of the muscle. Increasing the weight increases the length of the latent 

 period. 



Repeated stimulation, especially when the foot is weighted, is 

 followed by fatigue of the muscle. At first the work seems gradually 

 to increase the power of the contractions, and they progress to a 

 maximum ; but then fatigue begins and the power gradually diminishes 

 until the muscle becomes exhausted. 



Within certain limits, diminished temperature at first increases the 

 vigor of muscular contractions, which afterward are gradually dimin- 

 ished. Moderate increase of temperature increases the vigor of the 

 contractions and shortens all the periods, including the latent period. 



In an entire muscle, the contraction is in the form of a wave. With 

 delicate chronometric apparatus, the rapidity of this wave between two 

 points in a muscle may be measured. In the frog's muscle, the wave 

 moves at the rate of about 120 inches (3 meters) in a second. 



