over 30,000,000 times in a year, and if it once stops, all is over.

In the skin are wonderfully varied and complex organs--for instance, over
2,000,000 perspiration glands, which regulate the temperature and
communicate with the surface by ducts, which have a total length of some
ten miles.

Think of the miles of arteries and veins, of capillaries and nerves; of
the blood, with the millions of millions of blood corpuscles, each a
microcosm in itself.

Think of the organs of sense,--the eye with its cornea and lens, vitreous
humor, aqueous humor, and choroid, culminating in the retina, no thicker
than a sheet of paper, and yet consisting of nine distinct layers, the
innermost composed of rods and cones, supposed to be the immediate
recipients of the undulations of light, and so numerous that in each eye
the cones are estimated at over 3,000,000, the rods at over 30,000,000.

Above all, and most wonderful of all, the brain itself. Meinert has
calculated that the gray matter of the convolutions alone contains no less
than 600,000,000 cells; each cell consists of several thousand visible
atoms, and each atom again of many millions of molecules.

And yet with reasonable care we can most of us keep this wonderful
organization in health; so that it will work without causing us pain, or
even discomfort, for many years; and we may hope that even when old age
comes

"Time may lay his hand
Upon your heart gently, not smiting it