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.