THE MUSCLES. 555 



DURATION OF TETANUS. A tetanized muscle cannot be kept con- 

 tracted for a considerable length of time, even though the stimuli 

 be kept constant. The muscle begins to elongate at first some- 

 what quickly, but later more slowly. This change is produced by 

 fatigue of the muscle. 



Effect of Temperature on Muscle-curve. Low temperature makes 

 the contraction longer and lower; the latent period is longer, and 

 the relaxation-curve is greatly not unlike that of a fatigued muscle. 

 When the temperature is raised, the setting free of energy is more 

 rapid; hence the time of contraction is shortened, especially the 

 latent period and time of shortening of the muscle. 



Strength of Stimulus. If you apply a current just sufficient to 

 cause a muscle to contract, and then increase the strength . of the 

 current, the muscular contraction will become more rapid and more 

 complete. But the increase in contraction is not proportional to the 

 increase in stimulus. As the stimulus is gradually increased, the 

 increase in contraction becomes smaller and smaller. After a cer- 

 tain strength of stimulus is attained, a further increase of it does 

 not cause any increment in the contraction of the muscle. 



Amount of Load. If a muscle is attached to a lever without any 

 weight in the scale-pan, it is ascertained that light weights actually 

 increase the height of the contraction, whilst heavier weights dimin- 

 ish it until a limit is reached, and when a sufficient weight is used 

 the muscle no longer contracts. 



Muscle-tonus. This is a condition of a muscle more or less 

 stretched, and is dependent upon the reflex activity of the central 

 nervous system and a sufficient supply of blood to the muscle. If 

 you cut a motor nerve going to a muscle, the muscle loses its tonus. 

 [f you divide all the posterior spinal roots, then the muscles also lose 

 leir tonus. 



Muscle-sound. Helmholtz said that 36 vibrations per second 

 formed the average for the production of muscular tones. To-day 

 lis is considered an overtone, and the requisite number of necessary 

 dbrations is placed at 19 per second. 



FIRST HEART-SOUND. It is probable that the first sound of the 

 irt is partly a muscle-sound. It is a dull sound, persisting when 

 ic thorax is taken away and the auriculo-ventricular valves are de- 

 )yed. The sound could not in such an instance be produced by 

 le vibration of the valves. 



Voluntary Contraction. The number of single impulses sent to 

 our muscles during voluntary movements are somewhat variable. 



