The summation of sub-threshold excitation to a certain height offers very favorable conditions for the development of _tonus_. (Figure 54.) This fact has been established for many kinds of centers (cardio-inhibitory center, vasomotor center, etc.). During the continuance of a series of stimuli, as we have already seen, an equilibrium between disintegration and replacement soon takes 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 _Thörner_[188] 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 _Thörner_ 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 perceptible 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. _Fröhlich_[189] 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 interference dare not be overlooked in the genesis of muscle tonus. If it is true, as appears from the investigations of _Keith Lucas_,[190] that the “all or none law” applies to striated muscle, then an increase of the contraction from homotopic summation cannot occur, because an isobolic system cannot show an increase of its already maximal excitation by summation. Such being the case, the tonic shortening of striated muscle can only be explained as an expression of a heterotopic interference. [188] _Thörner_: “Weitere Untersuchungen über die Ermüdung des markhaltigen Nerven. Die Ermüdung in Luft.” Zeitschr. f. allgem. Physiologie Bd. X, 1910.