the laws of rhythm in disease. Certain fevers have their regular periods of attack, as intermittent fever; while all diseases have their periods of exacerbation, however invariable the symptoms may seem to be to the ordinary observer or even to the patient himself.
Doubtless the fact that certain hereditary diseases do not appear in the offspring at once, but only at the age at which they were manifested in the parents, is owing to the same cause.
Let us now examine more thoroughly into the real nature of this rhythm which pervades the entire universe.
If a bow be drawn across a violin-string on which some small pieces of paper have been placed, these will be seen to fly off; and if the largest string be experimented upon, it can be observed to be in rapid to-and-fro motion, known as vibration, which motion is perfectly regular, a definite number of movements occurring within a measured period of time; in other words the motion is rhythmical. In strings of the finest size the motion is not visible, but we judge of its existence because of the result, which is in each instance a sound. Sound is to us, however, an affection of the nerve of hearing and the brain, owing to the vibrations of the ear caused by similar vibrations of the violin-strings. The movements of the nerves and nerve-cells are invisible and molecular, and we seem to be justified in regarding molecular movements as constant and associated with all the properties of matter whether living or dead.
We see, then, that all things living and lifeless are in constant motion, visible or invisible; there is no such thing in the universe as stable equilibrium. Change, ceaseless change, is written on all things; and, so far as we can judge, these changes, on the whole, tend to higher development. Neither rhythm, however, nor anything else, is perfect. Even the motions of planets are subject to perturbations or irregularities in their periodicity. This subject is plainly boundless in its scope. We have introduced it at this stage to prepare for its study in detail in dealing with each function of the animal body. If we are correct as to the universality of the law of rhythm, its importance in biology deserves fuller recognition than it has yet received in works on physiology; it will, accordingly, be frequently referred to in the future chapters of this book.
