well-developed muscles, large nerves, and a full-sized,
healthy, and active brain.

_Observation._ The muscles of fishes are large, and the nerves
distributed to them, comparatively small. The muscles of birds are
small, but their fibres are very compact. The nerves appropriated to
the muscles that are called into action in flying, are large as well
as numerous.

171. The contractile portion of a muscle is, in general, at a distance
from the part to be moved. Thus the principal muscles that move the
fingers are situated upon the forearm; and when the limb is nearly or
quite extended, the angle formed by the part to be moved and the
contractile muscles is small. Again, the attachment of the muscles to
the part to be moved is near the joint that forms the fulcrum, (fig.
45.) By these arrangements there is a loss of power; but we are
compensated for this disadvantage by increased celerity of movement,
beauty of form, and adaptation of the limbs to the varied pursuits of
man.

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170. What is said of those persons who have small muscles and largely
developed nervous systems? Of those who have large muscles and small
nerves? Upon what do strength and the power of endurance depend? 171.
Why is there a loss of power in the action of the muscles?

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_Illustration._ The muscle that bends the elbow acts at disadvantage,
and this is greatest when the arm is nearly or quite extended, as the
angle of action is then least. This disadvantage is further increased
by the attachment of the motive muscles near the joint.

172. The number of muscles which are called into action in the
movements of the different joints, varies. The hinge-joints, as the
elbow, have two sets of muscles--one to bend the joint, the other to
extend it. The ball and socket joints, as the shoulder, are not
limited to mere flexion and extension. No joint in the system has the
range of movement that is possessed by that of the shoulder. By the
action of the muscles attached to the arm, it is not only carried
upward and forward, but forward and backward. Hence the arm may be
moved at any angle, by a combined action of its muscles.

_Observation._ "Could we behold properly the muscular fibres in
operation, nothing, as a mere mechanical exhibition, can be conceived
more superb than the intricate and combined actions that must take
place during our most common movements. Look at a person running or
leaping, o