ufficiently compressed to drive out all the
air. Air is always present in the lungs. This keeps them more or less
distended and pressed against the thoracic walls.

*How the Thoracic Space is Varied.*--One means of varying the size of the
thoracic cavity is through the movements of the ribs and their resultant
effect upon the walls of the thorax. In bringing about these movements the
following muscles are employed:

1. The _scaleni_ muscles, three in number on each side, which connect at
one end with the vertebrae of the neck and at the other with the first and
second ribs. Their contraction slightly raises the upper portion of the
thorax.

2. The _elevators of the ribs_, twelve in number on each side, which are
so distributed that each single muscle is attached, at one end, to the
back portion of a rib and, at the other, to a projection of the vertebra a
few inches above. The effect of their contraction is to' elevate the
middle portion of the ribs and to turn them outward or spread them apart.

3. The _intercostal_ muscles, which form two thin layers between the ribs,
known as the _internal_ and the _external_ intercostal muscles. The
external intercostals are attached between the outer lower margin of the
rib above and the outer upper margin of the rib below, and extend
obliquely downward and forward. The internal intercostals are attached
between the inner margins of adjacent ribs, and they extend obliquely
downward and backward from the front. The contraction of the external
intercostal muscles raises the ribs, and the contraction of the internal
intercostals tends to lower them.

[Fig. 42]

Fig. 42--*Simple apparatus* for illustrating effect of movements of the
ribs upon the thoracic space; strips of cardboard held together by pins,
the front part being raised or lowered by threads moving through
attachments at 1 and 2. As the front is raised the space between the
uprights is increased. The front upright corresponds to the breastbone,
the back one to the spinal column, the connecting strips to the ribs, and
the threads to the intercostal muscles.

By slightly raising and spreading apart the ribs the thoracic space is
increased in two directions--from front to back and from side to side.
Lowering and converging the ribs has, of course, the opposite effect (Fig.
42). Except in forced expirations the ribs are lowered and converged by
their own we