not work with forks of different pitch.)
2. While holding a thin piece of paper against a comb with the open lips,
produce musical tones with the vocal cords. These will set the paper in
vibration, producing the so-called "comb music." 3. Examine the disk in a
telephone which is set in vibration by the voice. Observe that it is a
thin disk and, like the membrane of the ear, has air on both sides of it.

*To show the Reenforcement of Sound.*--1. Vibrate a tuning fork in the air,
noting the feebleness of the tone produced. Then hold the stem against a
door or the top of a table, noting the difference. 2. Hold a vibrating
tuning fork over a tall jar, or bottle, and gradually add water. If the
vessel is sufficiently tall, a depth will be reached where the air in the
vessel reenforces the sound from the fork. 3. Hold a vibrating fork over
the mouth of a small fruit jar, partly covered with a piece of cardboard.
By varying the size of the opening, a position will be found where the
sound is reenforced. If not successful at first, try bottles and jars of
different sizes.

*To illustrate the Manner of Vibration of the Liquid in the Internal
Ear.*--Tie a piece of dental rubber over the end of a glass or wooden tube
about half an inch in diameter and six inches in length. Fill the tube
entirely full of water and, without spilling, tie a piece of thin rubber
tightly over the other end. Holding the tube horizontally, press the
rubber in at one end and note that it is pushed out at the other end. Make
an imitation of a vibration with the finger against the rubber at one end
of the tube and note the effect at the other end. To what do the tube and
the rubber on the ends of the tube correspond in the internal ear?

[Fig. 156]

Fig. 156--*Simple apparatus* for demonstrating the larynx.

*To show the Plan of the Larynx.*--Cut from stiff paper four pieces of
different shapes as indicated in Fig. 156. (The piece to the left should
have a length of about six inches, the others proportionally large.) The
largest represents the thyroid cartilage, the next in size the cricoid,
and the two smallest the arytenoid cartilages. By means of pins, or
threads, connect these with each other according to the description of the
larynx on page 253. With this simple model the movements of the different
cartilages and their effect upon the vocal cords may be illustrated.

*To show the Relation of the Movemen