And yet in that body we have more than 200 bones, of complex and varied
forms, any irregularity in, or injury to, which would of course grievously
interfere with our movements.
We have over 500 muscles; each nourished by almost innumerable blood
vessels, and regulated by nerves. One of our muscles, the heart, beats
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.
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