42 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



to an increasing^ extent as the second contraction falls later and 

 later in the period of relaxation after the first contraction.* If 

 instead of two we use three successive stimuli, falling into the muscle 

 at proper intervals, a still further summation occurs. In this way 

 the total extent of shortening in a muscle completely tetanized may 

 be several times as great as that of a single maximal contraction. 



The Discontinuous Character of the Tetanic Contraction 

 The Muscle-tone. In complete tetanus the muscle seems to 

 be in a condition of continuous uniform contraction; the re- 

 corded curve shows no sign of relaxation between stimuli and no 

 external indication, in fact, that the separate stimuli do more than 

 maintain a state of uniform contraction. It can be shown, how- 

 ever, that in reality each stimulus has its own effect, and that the 

 chemical changes underlying the phenomenon of contraction are 

 probably not continuous, but form an interrupted series correspond- 

 ing, within limits, to the series of stimuli sent in. The clearest 

 proof for this belief is found in the electrical changes that result 

 from each stimulus, and the facts relating to this side of the question 

 will be stated subsequently in the chapter on The Electrical 

 Phenomena of Muscle and Nerve. Another proof is found in the 

 phenomenon of the muscle-tone. When a muscle is stimulated 

 directly or through its motor nerve a musical note may be heard 

 by applying the ear or a stethoscope to the muscle. -The note that 

 is heard corresponds in pitch, up to a certain point, with the num- 

 ber of stimuli sent in, that is, the muscle vibrates, as it were, in 

 unison with the number of stimuli, and, although the vibrations 

 are not sufficient to affect the recording lever, they can be heard 

 as a musical note. This fact, therefore, may be taken as a proof 

 that during complete tetanus there is a discontinuous series of 

 changes in the muscle the rate of which corresponds with that of the 

 stimulation. The series of electrical changes corresponding with the 

 series of stimuli sent in may be made audible by applying a telephone 

 to the muscle. Making use of this method, Wedenskif has shown 

 that the ability of the muscle to respond isorhythmically to the 

 rate of stimulation is limited. In frog's muscle the pitch of the 

 musical tone may correspond with the rate of stimulation up to 

 about 200 stimuli per second. In the muscle of the warm-blooded 

 animal the correspondence may extend to about 1000 stimuli per 

 second. If the rate of stimulation is increased beyond these 

 limits the musical note heard does not correspond, but falls 

 to a lower pitch, indicating that some of the stimuli under these 

 conditions become ineffective. It should be added that the high 



* Von Kries, "Archiv fur Physiologic," 1888, p. 537. 



fWedenski, "Du rhythme musculaire dans la contraction normale," 

 "Archives de physiologic," 1891, p. 58. 



