n. Similar, though smaller, sacs are found over the
point of the elbow, over the knuckles, the ankle bones, and various other
prominent points. These sacs answer a very important purpose, and are
liable to various forms of inflammation.

Experiment 21. Examine carefully the tendons in the parts dissected
in Experiment 18. Pull on the muscles and the tendons, and note how they
act to move the parts. This may be also admirably shown on the leg of a
fowl or turkey from a kitchen or obtained at the market.

Obtain the hoof of a calf or sheep with one end of the tendon of
Achilles still attached. Dissect it and test its strength.

73. Mechanism of Movement. The active agents of bodily movements, as
we have seen, are the muscles, which by their contraction cause the bones
to move one on the other. All these movements, both of motion and of
locomotion, occur according to certain fixed laws of mechanics. The bones,
to which a great proportion of the muscles in the body are attached, act
as distinct levers. The muscles supply the power for moving the
bones, and the joints act as fulcrums or points of support. The weight of
the limb, the weight to be lifted, or the force to overcome, is the
resistance.

74. Levers in the Body. In mechanics three classes of levers are
described, according to the relative position of the power, the fulcrum,
and the resistance. All the movements of the bones can be referred to one
or another of these three classes.

Levers of the first class are those in which the fulcrum is between
the power and the weight. The crowbar, when used to lift a weight at one
end by the application of power at the other, with a block as a fulcrum,
is a familiar example of this class. There are several examples of this in
the human body. The head supported on the atlas is one. The joint between
the atlas and the skull is the fulcrum, the weight of the head is the
resistance. The power is behind, where the muscles from the neck are
attached to the back of the skull. The object of this arrangement is to
keep the head steady and balanced on the spinal column, and to move it
backward and forward.

[Illustration: Fig. 35.--Showing how the Bones of the Arm serve as Levers.

P, power;
W, weight;
F, fulcrum.
]

Levers of the second class are those in which the weight is between
the fulcrum and the power. A familiar example is the crowbar when used for
lifting a weight while one end rests on the ground