f the superficial layer of muscles on the anterior
portion of the body.]

[Illustration: Fig. 25.
A representation of the superficial layer of muscles on the posterior
portion of the body.]

Very rarely is motion produced by the action of a single muscle, but by
the harmonious action of several. There is infinite variety in the
arrangement of the muscles, each being adapted to its purpose, in
strength, tenacity, or elasticity. While some involuntarily respond to
the wants of organic life, others obey, with mechanical precision, the
edicts of the will. The peculiar characteristic of the muscles is their
contractility; for example, when the tip of the finger is placed in the
ear, an incessant vibration, due to the contraction of the muscles of
the ear, can be heard. When the muscles contract, they become shorter;
but what is lost in length is gained in breadth and thickness, so that
their actual volume remains the same. Muscles alternately contract and
relax, and thus act upon the bones. The economy of muscular power thus
displayed is truly remarkable. In easy and graceful walking, the forward
motion of the limbs is not altogether due to the exercise of muscular
power, but partly to the force of gravity, and only a slight assistance
of the muscles is required to elevate the leg sufficiently to allow it
to oscillate.

Motion is a characteristic of living bodies. This is true, not only in
animals, but also in plants. The oyster, although not possessing the
power of locomotion, opens and closes its shell at pleasure. The coral
insect appears at the door of its cell, and retreats at will. All the
varied motions of animals are due to a peculiar property of the muscles,
termed _contractility_. Although plants are influenced by external
agents, as light, heat, electricity, etc., yet it is supposed that they
may move in response to inward impulses. The sensitive stamens of the
barberry, when touched at their base on the inner side, resent the
intrusion, by making a sudden jerk forward. Venus's fly-trap, a plant
found in North Carolina, is remarkable for the sensitiveness of its
leaves; which close suddenly and capture insects which chance to alight
upon them. The muscles of the articulates are situated within the solid
framework, unlike the vertebrates, whose muscles are external to the
bony skeleton. All animals have the power of motion, from the lowest
radiate to the highest vertebrate, from the most repulsive polyp to that