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"What's yet in this
That bears the name of life? Yet in this life
Lie hid more thousand deaths."

Measure for Measure, Act iii. Scene 1.

No physical or psychical act is possible without this change. It is a process of continual waste and repair. Subject to its inevitable power, the organization is continually wasting away and continually being repaired.

The old notion that our bodies are changed every seven years, science has long since exploded. "The matter," said Mr. John Goodsir, "of the organized frame to its minutest parts is in a continual flux." Our bodies are never the same for any two successive days. The feet that Mary shall dance with next Christmas Eve will not be the same feet that bore her triumphantly through the previous Christmas holidays. The brain that she learns German with to-day does not contain a cell in its convolutions that was spent in studying French one year ago. Whether her present feet can dance better or worse than those of a year ago, and whether her present brain can do more or less German and French than the one of the year before, depends upon how she has used her feet and brain during the intervening time, that is, upon the metamorphosis of her tissue.

From birth to adult age, the cells of muscle, organ, and brain that are spent in the activities of life, such as digesting, growing, studying, playing, working, and the like, are replaced by others of better quality and larger number. At least, such is the case where metamorphosis is permitted to go on normally. The result is growth and development. This growing period or formative epoch extends from birth to the age of twenty or twenty-five years. Its duration is shorter for a girl than for a boy. She ripens quicker than he. In the four years from fourteen to eighteen, she accomplishes an amount of physiological cell change and growth which Nature does not require of a boy in less than twice that number of years. It is obvious, that to secure the best kind of growth during this period, and the best development at the end of it, the waste of tissue produced by study, work, and fashion must not be so great that repair will only equal it. It is equally obvious that a girl upon whom Nature, for a limited period and for a definite purpose, imposes so great a physiological task, will not have as much power left for the tasks of the school, as the boy of whom Nature requires less at the corresponding epoch. A margin must be allowed for growth. The repair must be greater and better than the waste.

During middle age, life's active period, there is an equilibrium between the body's waste and repair: one equals the other. The machine, when properly managed, then holds its own. A French physiologist fixes the close of this period for the ideal man of the future at eighty, when, he says, old age begins. Few have such inherited power, and live with such physiological wisdom, as to keep their machine in good repair, -- in good working-order, -- to that late period. From the age of twenty-five or thirty, however, to that of sixty or sixty-five, this equilibrium occurs. Repair then equals waste; reconstruction equals destruction. The female organization, like the male, is now developed: its tissues are consolidated; its functions are established. With decent care, it can perform an immense amount of physical and mental labor. It is now capable of its best work. But, in order to do its best, it must obey the law of periodicity; just as the male organization, to do its best, must obey the law of sustained effort.

When old age begins, whether, normally, at seventy or eighty, or, prematurely, at fifty or thirty, repair does not equal waste, and degeneration of tissue results. More cells are destroyed by wear and tear than are made up from nutriment. The friction of the machine rubs the stuff of life away faster than it can be replaced. The muscles stiffen, the hair turns white, the joints crack, the arteries ossify, the nerve-centres harden or soften: all sorts of degeneration creep on till death appears, -- Mors janua vitæ. There the curves unite, and men and women are alike again.

Sleep, whose inventor received the benediction of Sancho Panza, and whose power Dryden apostrophized, --

"Of all the powers the best:
Oh! peace of mind, repairer of decay,
Whose balm renews the limbs to labor of the day," --

is a most important physiological factor. Our schools are as apt in frightening it away as our churches are in inviting it. Sleep is the opportunity for repair. During its hours of quiet rest, when muscular and nervous effort are stilled, millions of microscopic cells are busy in the penetralia of the organism, like coral insects in the depths of the sea, repairing the waste which the day's study and work have caused. Dr. B.W. Richardson of London, one of the most ingenious and accomplished physiologists of the present day, describes the labor of sleep in the following language: "During this period of natural sleep, the most important changes of nutrition are in progress: the body is renovating, and, if young, is actually growing. If the body be properly covered, the animal heat is being conserved, and laid up for expenditure during the waking hours that are to follow; the respiration is reduced, the inspirations being lessened in the proportion of six to seven, as compared with the number made when the body is awake; the action of the heart is reduced; the voluntary muscles, relieved of all fatigue, and with the extensors more relaxed than the flexors, are undergoing repair of structure, and recruiting their excitability; and the voluntary nervous system, dead for the time to the external vibration, or, as the older men called it, 'stimulus' from without, is also undergoing rest and repair, so that, when it comes again into work, it may receive better the impressions it may have to gather up, and influence more effectively the muscles it may be called upon to animate, direct, control." (11) An American observer and physiologist, Dr. William A. Hammond, confirms the views of his English colleague. He tells us that "the state of general repose which accompanies sleep is of especial value to the organism, in allowing the nutrition of the nervous tissue to go on at a greater rate than its destructive metamorphosis." In another place he adds, "For the brain, there is no rest except during sleep." And, again, he says, "The more active the mind, the greater the necessity for sleep; just as with a steamer, the greater the number of revolutions its engine makes, the more imperative is the demand for fuel." (12) These statements justify and explain the instinctive demand for sleep. They also show why it is that infants require more sleep than children, and children than middle-age folk, and middle-age folk than old people. Infants must have sleep for repair and rapid growth; children, for repair and moderate growth; middle-age folk, for repair without growth; and old people, only for the minimum of repair. Girls, between the ages of fourteen and eighteen, must have sleep, not only for repair and growth, like boys, but for the additional task of constructing, or, more properly speaking, of developing and perfecting then, a reproductive system, -- the engine within an engine. The bearing of this physiological fact upon education is obvious. Work of the school is work of the brain. Work of the brain eats the brain away. Sleep is the chance and laboratory of repair. If a child's brain-work and sleep are normally proportioned to each other, each night will more than make good each day's loss. Clear heads will greet each welcome morn. But if the reverse occurs, the night will not repair the day; and aching heads will signalize the advance of neuralgia, tubercle, and disease. So Nature punishes disobedience.

(12) Sleep and its Derangements, pp. 9, 10, 13.