5. The period composed of a number of unit groups (the verse, in nonsense syllables) has a general form which suggests strongly that it has the unity of a single coördinated movement. There is no more reason for assuming a transcendental mental activity in the case of a rhythmic period than in the case of a single act which appears in consciousness as a unity. Undoubtedly the breathing is correlated with the rhythmic movements and may be a factor in determining the verse period. Meumann's principal accent, about which a number of subordinate accents are grouped, is characteristic not only of poetry but of the simplest rhythms. At some point in the period there is a definite climax, a chief accent; the movement 'rises' to that point and then falls off. This is strikingly seen in nonsense verses spoken with a heavy accent within the verse. The accent does not stand out from a dead level, but the verse culminates at that point.
Unfortunately very little is known of the mechanism of so simple a coördinated muscular activity as that necessary for a simple rhythm. Sherrington[17] and Hering[18]have pointed out the primary character of the grouping of the muscles in opposing sets and the reciprocal nature of almost all muscular activity, but in a review of the work of coördinated movements Hering denies any simultaneous stimulation of the two sets and considers the question of the innervation mechanism of opposing muscle-sets entirely unsettled.
That the connection between the positive and negative set of muscles in a rhythmic movement is very close, and that the reaction is of the circular type, is evident from the automatic character of all rhythmic movements, and it is evident that the limiting sensation is the primary cue in the reaction. Anything further is mere hypothesis. Robert Müller's[19] thorough criticism of the Mosso ergograph throws great doubt on the present methods of investigation and invalidates conclusions from the various curves of voluntary movements which have been obtained.
The curve of contraction and relaxation of a simple muscle is well known and is not affected by Müller's criticism. Its chief characteristic, with or without opposing tension, is the inequality of the intervals of the contraction and relaxation phases. As one might expect, since a single set of muscles dominates in a rhythmic movement, the typical rhythmic curve has the general character of the curve of the simple muscle. The average values of the phases of curves of simple rhythmic movement obtained by A. Cleghorn[20] from a large number of observations with at least three subjects, are as follows: phase of contraction, .44 second; phase of relaxation, .54 second. It is very significant for a motor theory of rhythm that this general form of the curve of rhythmic movement may easily be altered in all sorts of fashions by unusual stimuli to the two muscle sets.
While it is well recognized that a rhythm does not consist necessarily of sound sensations, the 'rhythmization' of a series of sound sensations in the ordinary perceived rhythms is a matter of great interest. Ewald found strong reasons for believing that the ear is peculiarly connected with the motor apparatus. The experiments of Hofbauer[21] and Cleghorn[22]show that any strong stimulus to either eye or ear modifies decidedly the reactions of coördinated muscles. How shall we assume that the automatic movement cycle necessary to rhythmic perception is set up when one listens to a series of sounds?
It must be assumed that any chance sound sets up a contraction in a set of muscles, however large or small. If but a single sound occurs, the phase of contraction in that muscle set is followed by a longer phase of relaxation, and the musculature is passive as before; it may be that the stretching of the antagonistic set of muscles weakly stimulates them, and they then contract during the relaxation phase and assist in restoring the original condition.
But if a second sound occurs toward the end of the relaxation phase, before the tension is quite exhausted, the movement will be repeated; the negative set of muscles will be more definitely stimulated, for the activity will not have been exhausted when the second sound occurs. If the sound continues to recur at regular intervals, the movement cycle thus established will rapidly become coördinated. The positive set in its vigorous contraction furnishes a limiting sensation which becomes a cue for its own relaxation and for the reciprocal contraction of the negative muscle set. The contraction of the negative muscle set and the resulting changes in tension may become in turn a cue for the positive set. The reaction is now of the circular type and the process has become self-regulative, though constantly reinforced by the recurring sound (which has become a part of the limiting sensation of the rhythmic movement cycle).
But it is very probable that the second sound may not be timed so as to come at the close of the relaxation phase in the set of muscles roused; moreover, in almost all rhythms there are secondary sounds occurring between the main beats. What happens when a sound occurs out of place, early in the phase of relaxation, or just before or just after the climax in the contraction phase? Does it make it impossible to establish the coördination, or destroy it if already established?
Hofbauer demonstrated that a stimulus which appears in close proximity to the limiting sensation, either before or after, always increases the force of the reaction, so that such a slight displacement could not affect the rhythm, which would quickly readjust itself. The possibility of a stimulus occurring in the relaxation phase is of much more importance for a motor theory of the initiation of a rhythmic movement. Cleghorn made the stimulus occur at the beginning of the relaxation phase. Instead of prolonging or reinstating the contraction phase, he found that the stimulus intensified the relaxation process and shortened its period. "The stimulated relaxation is not only quicker than the normal, but also more complete; the end of the normal relaxation is slow; ... relaxation under the influence of the stimulus, on the contrary, shows nothing of this, but is a sudden sharp drop directly to the base line and sometimes below it." A comparison of the normal phases with the same phases, when the stimulus occurs within the relaxation phase, follows:
Normal: Contraction-phase, .44 sec.; relaxation-phase, .54 sec.; total, .98 sec.