ich are found in animals on removing the skin. They constitute the red
fleshy part of meat, and give form and symmetry to the body. In the
limbs they surround and protect the bones, while in the trunk they
spread out and constitute a defensive wall for the protection of the
vital parts beneath. The muscles have been divided into _three parts_,
of which the middle and fleshy portion, called the _belly_, is most
conspicuous. The other two parts are the opposite ends, and are commonly
called the _origin_ and _insertion_ of the muscle. The _origin_ is
usually fastened to one bone, and the _insertion_ is attached to
another. By the contraction of the _belly_ of the muscle, the
_insertion_, which is _movable_, is drawn toward the _origin_, which is
_fixed_, and brings with it the bone to which it is attached. This any
one can see illustrated in bending the arm. The muscle which performs
this function lies between the elbow and the shoulder. It is attached to
the shoulder by its _origin_, and to one of the bones of the fore-arm,
just below the elbow, by its _insertion_. By grasping the arm midway
between the shoulder and the elbow with the opposite hand, and then
bending the arm, the enlargement of the belly of the muscle by the
contraction will be at once perceived. Then, by moving the hand down on
the inside of the arm toward the elbow, the lessening muscle may be
readily traced until it terminates in a _tendon_, of much less size than
the muscle, but of great strength, which is inserted into the bone just
below the elbow. As the fore-arm is drawn up, and especially if there be
a weight in the hand, the _tendon_ may be felt just within the
elbow-joint, running toward the point of insertion. Extend the arm at
the elbow, and the muscle on the outside of the arm will swell and
become firm, while the inside muscle, and its tendon at the elbow, will
be relaxed. This example well illustrates the principle on which all the
joints of the system are moved. Those who are acquainted with mechanics
will readily perceive that the action just described is an example of
the "_third_ kind of lever," where the power is applied between the
weight and the fulcrum. The elbow is the fulcrum, the hand contains the
weight, and the tendon, inserted into the bone just below the elbow, is
the power. This kind of lever requires the power to be greater than the
weight, and acts under what is called a _mechanical disadvantage_. What
is lost in power, howe