rious inflections, without injuring the nerves, or diminishing
that strength which is so much required.

The whole system of bones, or skeleton, is constructed of several
parts, of different shapes and sizes, joining with one another in
various manners, and so knit together, as best to answer to the
motions which the occasions of the animal may require.

These bones serve as levers for the muscles to act on; which last
serve as mechanical powers, to give the machine various motions,
at the command of the will.

The muscles are fleshy fibres, attached by their extremities to
the bones. When the fibres shorten themselves, the two parts into
which the muscle is inserted are brought nearer; and, by this
simple contrivance, all the motions of animals are performed, and
their bodies carried from one place to another.

Joints are provided with muscles for performing the motions for
which they are adapted; every muscle pulling the bone, to which it
is attached, in its own particular direction. Hence the muscles
may be considered as so many moving forces, as was before hinted;
and their strength, the distance of their insertion from the
centre of motion, the length of the lever to which they are
attached, and the weight connected with it, determine the duration
and velocity of the motions which they produce. Upon these
different circumstances depend the different kinds of motion
performed by various animals, such as the force of their leap, the
extent of their flight, the rapidity of their course, and their
address in catching their prey.

Most of the muscles act upon the bones, so as to produce the
effects of a lever of the third kind, as it is termed by
mechanics, where the power acts between the centre of motion and
the weight; hence it has a mechanical disadvantage; as an instance
of this, the muscle which bends the forearm, is inserted about one
eighth or one tenth of the distance from the centre of motion that
the hand is, where the weight or resistance is applied; hence the
muscle must exert a force eight or ten times greater than the
weight to be raised. But this disadvantage is amply compensated by
making the limbs move with greater velocity; besides, if room had
been given for the muscles to act with greater advantage, the
limbs must have been exceedingly deformed and unwieldy. [1]

The muscles, in general, at least those which serve for voluntary
motion, are balanced by antagonists, by means of which they are