The mysterious process which physiologists call metamorphosis of tissue, or intestitial change, deserves attention in connection with our subject. It interests both sexes alike. Unless it goes on normally, neither boys, girls, men, nor women, can have bodies or brains worth talking about. It is a process, without which not a step can be taken, or muscle moved, or food digested, or nutriment assimilated, or any function, physical or mental, performed. By its aid, growth and development are carried on. Youth, maturity, and old age result from changes in its character. It is alike the support and the guide of health convalescence, and disease. It is the means by which, in the human system, force is developed, and growth and decay rendered possible. The process, in itself, is one of the simplest. It is merely the replacing of one microscopic cell by another; and yet upon this simple process hang the issues of life and death, of thought and power.
Carpenter, in his physiology, reports the discovery, which we owe to German investigation, "that the whole structure originates in a single cell; that this cell gives birth to others, analogous to itself, and these again to many future generations; and that all the varied tissues of the animal body are developed from cells."[9] A more recent writer adds, "In the higher animals and plants, we are presented with structures which may be regarded as essentially aggregates of cells; and there is now a physiological division of labor, some of the cells being concerned with the nutriment of the organism, whilst others are set apart, and dedicated to the function of reproduction. Every cell in such an aggregate leads a life, which, in a certain limited sense, may be said to be independent; and each discharges its own function in the general economy. Each cell has a period of development, growth, and active life, and each ultimately perishes; the life of the organism not only not depending upon the life of its elemental factors, but actually being kept up by their constant destruction and as constant renewal."[10] Growth, health, and disease are cellular manifestations. With every act of life, the movement of a finger, the pulsation of a heart, the uttering of a word, the coining of a thought, the thrill of an emotion, there is the destruction of a certain number of cells. Their destruction evolves or sets free the force that we recognize as movement, speech, thought, and emotion. The number of cells destroyed depends upon the intensity and duration of the effort that correlates their destruction. When a blacksmith wields a hammer for an hour, he uses up the number of cells necessary to yield that amount of muscular force. When a girl studies Latin for an hour, she uses up the number of brain-cells necessary to yield that amount of intellectual force. As fast as one cell is destroyed, another is generated. The death of one is followed instantly by the birth of its successor. This continual process of cellular death and birth, the income and outgo of cells, that follow each other like the waves of the sea, each different yet each the same, is metamorphosis of tissue. This is life. It corresponds very nearly to Bichat's definition that, "life is organization in action." The finer sense of Shakspeare dictated a truer definition than the science of the French physiologist,—
"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,—