g with a speed incalculably greater. In each case the movements
are determined by some cause from without which is termed by
physiologists a _stimulus_. The slightest movement of the thin
cover-glass placed over the drop of water in which an amoeba is
immersed, on a microscopic slide, suffices to act as a stimulus, and
serves much the same purpose as an electric shock to the muscles of a
man. In man an elaborate apparatus exists for the process known as
respiration, but in this and in all other cases the mechanism is
composed of what is known technically as _cells_, the latter being the
units of structure, the individual bricks of the building, so to
speak; and just as any edifice is made up of individual pieces some of
which differ from one another while others do not to any appreciable
extent, so is it with the body. The individual cells of a muscle are
alike in structure and function, but they differ widely from those of
a gland or secreting organ, as the liver. But it is to be ever
remembered that the statements with which we set out hold: that is,
that however cells may differ, they have in all animals certain
properties in common. Of the muscle-cell, the liver-cell, and the
one-celled animal we may affirm the same properties, but the
difference is that while all are secretory the liver-cell is eminently
so, and produces bile, which other cells do not; that while it is but
feebly contractile, or susceptible of change of form, the muscle-cell
is characterized by this property above all others.

The lower we descend in the animal scale the more simple are the
mechanisms by which results are attained. The one-celled animal may be
said to breathe with its whole body, while the man employs a large
number of muscles, not to speak, at present, of other arrangements.
But when a muscle is examined under the microscope, it is found to
consist of cells, each one of which is physiologically in all
essentials like an amoeba, so that we may say that a muscle or other
tissue or organ is really a sort of colony of cells of similar
structure and function, all working in harmony like a happy family. We
actually do find colonies of unicellular animals much like amoeba,
so that the muscle-cells and all other cells of the body may be
compared to amoeba and other one-celled animals.

But while in such unicellular creatures all functions are properties
of the individual cell, among higher forms _systems_ take the place of
the protoplas