468 MANUAL, OF PHYSIOLOGY. 



number of vibrations, however, does not produce any audible 

 note ; hence it has been supposed that the note we hear is really 

 the first overtone, and not the fundamental tone. When a muscle 

 is thrown into tetanus by a current interrupted by a tuning fork, 

 a tone is said to be produced which corresponds to the number of 

 vibrations of the fork which causes the interruption in the cur- 

 rent, and thus regulates the number of stimulations which the 

 muscle receives. If, however, a contraction of the muscle be 

 brought about by stimulating the spinal cord, with the same 

 apparatus for making and 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. 



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- 

 still by means of a tourniquet or a tightly applied bandage. 

 When removed from the body, a mammalian muscle soon ceases 

 to be irritable and dies, but its functional activity may be renewed 

 by passing an artificial stream of arterial blood through its ves- 

 sels, and an isolated muscle may thus be made to contract re- 

 peatedly for a considerable time. 



On the other hand, the muscle of a cold-blooded animal will 

 remain alive for a long time many hours if kept cool and 

 moist. When its functional activity is about to fade, it may be 

 revived by means of an artificial stream of blood being caused 

 to flow through its vessels, just as in the case of the mammalian 

 muscle. 



Common experience teaches us that even when well supplied 

 with blood our own muscles become fatigued after very prolonged 



