In comparison with the general rhythm of the impulses which controls the whole life, the beat-rhythm is very insignificant, for it plays an important part only in the higher animals. A certain beat, it is true, can be observed in all forward movements by animals, because, in these, antagonistic movements of the limbs release one another in regular alternation. But there is nothing to indicate that the beat according to which these movements are set going, is fixed in the interior. On the contrary, the ease with which the limbs of the animal adapt themselves, in their to and fro movement, to the difficulties presented by the ground, depends on the lack of an internally fixed beat. As a rule, the free extension of the limbs takes place quicker than the pushing back of the ground, by which the body is driven forward.
The rhythm of the gait is, in most cases, so well adapted to the condition of the ground, because the excitation flows to the "antagonist," only when that is actually on the stretch. That happens, however, only when the " agonist " has contracted, and its representatives have been " locked " against excitation, while those of the extended antagonist are " unlocked."
The rate at which the extension and contraction in the muscles of the limbs alternate, depends in considerable degree on the obstacles on the path at the time, obstacles which the muscles must overcome by their contraction. If an internal beat were trying here to regulate the movements, this would only add to the difficulties.
We find an interiorly determined beat only in those effector organs that have always the same obstacle to overcome, such as the hearts of vertebrates, the margin of the umbrella of jelly-fishes and the wings of insects. The hearts of invertebrates move according to the general law of tension ; their beat, therefore, is not in constant dependence on the amount of blood they contain. The stroke of the wing in a bird is regulated all the time by the receptors, while in insects it is only the beginning and the end of the rhythmical wing-beat that depend on these ; the rhythm itself is quite automatically exerted from the wing-muscle centres.
The effector organs which preserve their own beat-rhythm have for this an arrangement of their own, which is expressed in the so-called "refractory period." The refractory period rhythmically lowers to zero the threshold-value for the regular waves of excitation flowing to the points where the nerves enter the muscles. In what the arrangement consists has not yet been ascertained.