52 HUMAN PHYSIOLOGY. 



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 contraction depends 

 upon the condition of the muscle, the load, the strength of stimulus, etc. 



Contraction of Non-striated Muscle. The curve obtained by regis- 

 tration of the contraction of non-striated muscle shows that it is similar in 

 many respects to that of the striated muscle, except that the duration of the 

 former is considerably longer than that of the latter. 



Action of Successive Stimuli. If a series of successive stimuli be applied 

 to a muscle, the effect will be different according to the rapidity with which 

 they follow one another. If the second stimulus be applied at the termina- 

 tion of the contraction due to the first stimulus, a second contraction follows, 

 similar in all respects to the first. A third stimulus produces a third contrac- 

 tion, and so on until the muscle becomes exhausted. If the second stimulus 

 be applied during either of the two periods of the first contraction, the effects of 

 of the two stimuli will be added together and the second contraction will add 

 itself to the first. The maximum contraction is obtained when the second 

 stimulus is applied -$ of a second after the first. 



Tetanus. When a series of stimuli are applied to a muscle, following 

 one another with median rapidity, the muscle does not get time to relax in 

 the intervals of stimulation, but remains in a state of vibratory contraction, 

 which may be regarded as incipient tetanus, or clonus. As the stimulation in- 

 creases in frequency, the vibrations become invisible, being completely fused 

 together. There is, nevertheless, during the tetanic condition a series of con- 

 tinuous contractions and relaxations taking place. After a varying length 

 of time the muscle becomes fatigued, and not withstanding the stimulation, 

 begins slowly to elongate. The number of stimuli necessary a second for 

 the production of tetanus varies in different animals e. g., 2 to 3 for muscles 

 of the tortoise; 10 for muscles of the rabbitts; 15 to 20 for the frog; 70 to 80 

 for birds; 330 to 340 for insects. 



