Look at yonder tree, examine
its parts, leaves, twigs, branches, trunk, all endowed with a common
life. Yet each little individual leaf lives and moves freely upon its
centre or twig, which is a common centre for many leaves. Many little
twigs in their turn, each free to move by itself within a certain limit,
are ranged along their common centre, a branch. Many branches cluster
around a large one, and all the largest branches in their turn cluster
around the common trunk, or great centre supporting the whole fabric.
Each leaf and twig and branch contributes its share to the life of the
whole tree, and is in turn supported by the general life and circulating
sap.
All this is repeated with far greater fulness and complexity in the
living animal, or in the human body. How numerous are the parts
composing a single organ! How many organs go to one system, how many
systems, bony, muscular, fibrous, circulatory, nervous, combine to make
up the entire body! Then again, all the members of the body move,
_within a certain limit_, in perfect independence of all the rest. The
finger can move without the hand, the hand can move without the arm, the
forearm without the upper arm, the entire arm without any other limb;
and yet all the parts of one limb, and all the limbs together, are
harmonized in action by the central brain.
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