218 IRRITABILITY 



place. The level of this state of equilibrium depends upon the 

 relative intensity of the stimuli. It is lower in the case of strong 

 and higher in that of weak stimuli. This fact becomes apparent 

 from the researches of Thorner^ on the fatigue of medullated 

 nerves in air. This investigator showed that during continued 

 tetanic stimulation of the nerve, the irritability fell to a certain 

 level, at which it remained so long as stimulation persisted. The 

 irritability decreased to a new level when the strength of the 

 stimulus was increased. These interesting experiments of Thor- 

 ncr show that the level reached when stimulation is continued is 

 higher as the intensity is weaker. It is, therefore, clear that this 

 level in summation of stimulation beneath the threshold can be 

 above that of the threshold of perceptible response, that is, a per- 

 ceptible tonic excitation may result. In the genesis of tonus in' 

 the muscle, there is another point to be taken into consideration. 

 Here we have a combination of a heterotopic interference with a 

 homotopic interference, for the total shortening of the muscle is 

 brought about in part by several contraction waves which occur 

 at various points at the same time and which follow each other, 

 therefore have a heterotopic sequence. If we consider a long 

 stretch of muscle, to one end of which a stimulus is applied, it will 

 be found that the contraction wave moves throughout the entire 

 length. If after a certain interval of time a second stimulus is 

 applied, the resultant wave moves along the muscle but does not 

 necessarily homotopically interfere with the first. In short, there 

 are two waves of contraction occurring coincidently in the muscle, 

 the muscle is now more strongly contracted. Frohlich^ has made 

 the fact intelligible by this means that tetanic shortening of a 

 muscle is greater than that of maximal shortening which can be 

 produced by strong single stimulation. This heterotopic inter- 

 ference dare not be overlooked in the genesis of muscle tonus. 

 If it is true, as appears from the investigations of Keith Lucas, ^ 



1 Thorner: "Weitere Untersuchungen fiber die Ermudung des markhaltigen Nerven. 

 Die Ermiidung in Luft." Zeitschr. f. allgem. Physiologie Bd. X, 1910. 



2 Fr. IV. Frohlich: "Ueber die scheinbare Steigerung," etc. Zeitschr. f. allgem. 

 Physiol. Bd. V, 1905. 



Z Keith Lucas: "On the gradation of activity in a skeletal muscle fiber." Journ. 

 of Physiology, Vol. XXXIII, 1905-06. The same: "The all or none law of contraction 

 of the skeletal muscle-fiber." Journ. of Physiology, Vol. XXXIII, 1909. 



