framework for the thorax is supplied by the ribs which connect with the
spinal column behind and with the sternum, or breast-bone, in front. They
form joints with the spinal column, but connect with the sternum by strips
of cartilage. The ribs do not encircle the cavity in a horizontal
direction, but slope downward from the spinal column both toward the front
and toward the sides, this being necessary to the service which they
render in breathing.

*How Air is Brought into and Expelled from the Lungs.*--The principle
involved in breathing is that air flows from a place of _greater_ to a
place of _less_ pressure. The construction of the thorax and the
arrangement of the lungs within it provide for the application of this
principle in a most practical manner. The lungs are suspended from the
upper portion of the thoracic cavity, and the trachea and the upper air
passages provide the only opening to the outside atmosphere. Air entering
the thorax must on this account pass into the lungs. As the thorax is
enlarged the air in the lungs expands, and there is produced within them a
place of _slightly less_ air pressure than that of the atmosphere on the
outside of the body. This difference causes the air to flow into the
lungs.

[Fig. 41]

Fig. 41--*Diagram illustrating the bellows principle in breathing.* _A._
The human bellows. _B._ The hand bellows. Compare part for part.

When the thorax is diminished in size, the air within the lungs is
slightly compressed. This causes it to become denser and to exert on this
account a pressure _slightly greater_ than that of the atmosphere on the
outside. The air now flows out until the equality of the pressure is again
restored. Thus the thorax, by making the pressure within the lungs first
slightly less and then slightly greater than the atmospheric pressure,
causes the air to move into and out of the lungs.

Breathing is well illustrated by means of the common hand bellows, its
action being similar to that of the thorax. It will be observed that when
the sides are spread apart air flows into the bellows. When they are
pressed together the air flows out. If an air-tight sack were hung in the
bellows with its mouth attached to the projecting tube, the arrangement
would resemble closely the general plan of the breathing organs (Fig. 41).
One respect, however, in which the bellows differs from the thorax should
be noted. The thorax is never s