means of which the diaphragm may be lowered.

[Illustration: Fig. 171.]

Such is a description of the essentials of the model. The difficulties
encountered in its construction are few and easily overcome. In the
first place, the cork must be air-tight, and it is best made so by
pouring a little melted paraffin over it, care being taken not to close
the tube. The rubber bags were taken from toy balloon-whistles.

In the construction of the diaphragm, it is to be remembered that it
also must be air-tight, and in order to resemble the human diaphragm, it
must have a conical appearance when at rest. In order to avoid making
any holes in the rubber, the two attachments (one of the rubber band,
and the other of the string) were made in this wise: the rubber was
stretched over a button having an eye, then under the button was placed
a smaller ring from an old umbrella; to this ring was attached the
rubber band, and to the eye of the button was fastened the operating
string. When not in use the diaphragm should be taken off to relieve the
strain on the rubber band."

Experiment 204. _To illustrate the action of the intercostal
muscles_ (see sec. 210). The action of the intercostal muscles is not at
first easy to understand; but it will be readily comprehended by
reference to a model such as that represented in Fig. 172. This maybe
easily made by the student himself with four laths of wood, fastened
together at the corners, A, B, C, D, with pins or small screws, so as
to be movable. At the points E, F, G, H, pins are placed, to which
elastic bands may be attached (A). B D represents the vertebral column;
A C, the sternum; and A B and C D, the ribs. The elastic band F G
represents the _external_ intercostal muscles, and E H, the _internal_
intercostals.

[Illustration: Fig. 172.]

If now the elastic band E H be removed, the remaining band, F G, will
tend to bring the two points to which it is attached, nearer together,
and the result will be that the bars A B and C D will be drawn upwards
(B), that is, in the same direction as the ribs in the act of
_inspiration_. When the elastic band E H is allowed to exert its force,
the opposite effect will be produced (C); in this case representing the
position of the ribs in an act of _expiration_.

Experiment 205. Pin a round piece of bright red paper (large as a
dinner-plate) to a white wall, with a single p