Page:A Text-book of Animal Physiology.djvu/67

Rh that their cessation would arouse the attention of the least instructed; food is demanded at regular intervals; the juices of the digestive tract are poured out, not constantly but periodically; the movements by which the food is urged along its path are markedly rhythmic; the chemical processes of the body wax and wane like the fires in a furnace, giving rise to regular augmentations of the temperature of the body at fixed hours of the day, with corresponding periods of greatest bodily activity and the reverse.

This principle finds perfect illustration in the nervous system. The respiratory act of the higher animals is effected through muscular movements dependent on regular waves of excitation reaching them along the nerves from the central cells which regularly discharge their forces along these channels. Were not the movements of the body periodic or rhythmical, instead of that harmony which now prevails, every muscular act would be a convulsion, though even in the movements of the latter there is a highly compounded rhythm, as a noise is made up of a variety of musical notes. The senses are subject to the same law. The eye ceases to see and the ear to hear and the hand to feel if continuously stimulated; and doubtless in all art this law is unconsciously recognized. That ceases to be art which fails to provide for the alternate repose and excitation of the senses. The eye will not tolerate continuously one color, the ear a single sound. Why is a breeze on a warm day so refreshing? The answer is obvious.

Looking to the world of animate nature as a whole, it is noticed that plants have their period of sprouting, flowering, seeding, and decline; animals are born, pass through various stages to maturity, diminish in vigor, and die. These events make epochs in the life-history of each species; the recurrence of which is so constant that the agricultural and other arrangements even of savages are planned accordingly. That the individuals of each animal group have a definite period of duration is another manifestation of the same law.

Superficial observation suffices to furnish facts which show that the same law of periodicity is being constantly exemplified in the world of inanimate things. The regular ebb and flow of the tides; the rise and subsidence of rivers; the storm and the calm; summer and winter; day and night—are all recurrent, none constant.

Events apparently without any regularity, utterly beyond any law of recurrence, when sufficiently studied are found to