470 MANUAL OF PHYSIOLOGY. 



tension of the tetanized muscle, some variation in tension must 

 occur, since a low humming sound like the purring of a cat is 

 produced during contraction. This muscle tone can be heard by 

 applying the ear firmly over any large muscle (biceps) while in 

 tetanus, or by throwing the muscles attached to the Eustachian 

 tube into action, as in swallowing, or during spasm of the muscles 

 in mastication. 



The number of vibrations which has been estimated for the 

 human skeletal muscles does not produce any audible note ; hence 

 it has been supposed that the note we hear -is the first overtone. 

 When a muscle is thrown into tetanus by a current interrupted 

 by a tuning-fork, a tone is produced which corresponds to the 

 number of vibrations of the fork which causes the interruption 

 in the current, and thus regulates the number of stimulations 

 which the muscle receives. If, on the other hand, a contraction 

 of the muscle be brought about by stimulating the spinal cord, 

 with the same rate of breaking the current, then the normal 

 muscle tone is produced, just as if the contraction was the result 

 of a nerve impulse coming from the brain. 



However, there is no satisfactory proof that a rhythmical vari- 

 ation of tension is an essential part of the voluntary contraction 

 of muscle. Since the pitch varies with the tension of the mem- 

 brana tympani, it has been suggested that the so-called muscle 

 tone is really the resonant tone proper to the membrane of the 

 drum, which is evoked by the trembling movements due to varia- 

 tions either of force or distribution of the stimulus. 



IRRITABILITY AND FATIGUE. 



The life of the muscle tissue of mammalian animals is closely 

 dependent upon a good supply of nutrition, and if its blood 

 current be completely cut off by any means for a length of time 

 it loses its power of contracting. While the muscle remains in 

 the body, and is therefore kept warm and moist by the juices in 

 the tissues, it will live a very considerable time without any blood 

 flowing through it, and it at once regains its contractility when 

 the blood stream is again allowed to flow through its vessels. 

 This is seen when the circulation of a limb is brought to a stand- 



