1. Lay the nerve over the ends of the wires from a small battery which are
attached to the support at _A_, and arrange a second break in the circuit
at _B_. At this place the battery circuit is made and broken either by a
telegraph key or by simply touching and separating the wires. Note that
the muscle gives a single contraction, or twitch, both when the current is
made and when it is broken.

2. Remove the current and pinch the end of the nerve, noting the result.
With very fine wires, connect the battery directly to the ends of the
muscle. Stimulate by making and breaking the current as before. In this
experiment the muscle cells are stimulated by the direct action of the
current and not by the current acting on the nerve.

3. With the wires attached to either the muscle or the nerve, make and
break the current in rapid succession. This causes the muscle to enter
into a second contraction before it has relaxed from the first, and if the
shocks follow in rapid succession, to continue in the contracted state.
This condition, which represents the method of contraction of the muscles
in the body, is called _tetanus_.

NOTE.--In these experiments a twitching of the muscle is frequently
observed when no stimulus is being applied. This is due to the drying out
of the nerve and is prevented by keeping it wet with a physiological salt
solution. (See footnote, page 38.)

*To show the Action of Levers.*--With a light but stiff wooden bar, a
spring balance, and a wedge-shaped fulcrum, show:

1. The position of the weight, the fulcrum, and the power in the different
classes of levers, and also the weight-arm and the power-arm in each case.

2. The direction moved by the power and the weight respectively in the use
of the different classes of levers.

3. That when the power-arm and weight-arm are equal, the power equals the
weight and moves through the same distance.

4. That when the power-arm is longer than the weight-arm, the weight is
greater, but moves through a shorter distance than the power.

5. That when the weight-arm is longer than the power-arm, the power is
greater and moves through a shorter distance than the weight.

*To show the Loss of Power in the Use of the Body Levers.*--Construct a
frame similar to, but larger than, that shown in Fig. 120, (about 12
inches high), and hang a small spring balance (250 grams capacity) at the
place where the muscle is attached. Fasten the end of a lever to the
upright pi