Section II. ACCESSORY STRUCTURES OF THE RESPIRATORY SYSTEM
All of the structures in this section play a part in the functioning of the
The diaphragm, the chief muscle of respiration, is a thin, but strong, dome-
shaped muscular membrane. It separates the abdominal and thoracic cavities. The
diaphragm is attached to the inferior margin of the rib cage and to the bodies of the
lumbar vertebrae behind. As a muscular membrane, it domes upward into the thoracic
cavity. Upon contraction, the fibers of the diaphragm shorten and pull downward. This
downward motion produces a piston-like pressure on the contents of the
a. The intercostal spaces are filled by two layers of intercostal muscles. The
intercostal muscles extend from the vertebrae behind to the sternum in front. A
strengthening "plywood effect" is created by the arrangement of the two layers at a right
angle to each other. These muscles help maintain the "solid-wall" condition of the
thorax. For this reason, a pressure gradient can be maintained between the inside and
outside of the thorax.
b. The intercostal muscles play a part in the mechanics of breathing. Quiet
breathing takes place due to the alternate contraction and relaxation of the diaphragm
and the internal intercostal muscles. As an individual breathes in, the diaphragm
contracts and, at the same time, the external intercostal muscles contract causing the
ribs to be pulled upward and the sternum to be pushed forward. This increases the
anterior-posterior diameter of the thoracic cavity. (The volume of the chest cavity
increases.) When the individual breathes out, the external intercostal muscles relax, the
ribs move downward, and, as the diaphragm relaxes, the thoracic cage moves upward.
These movements decrease the vertical and anterior-posterior diameters of the thoracic
cavity. The thoracic cavity (smaller in volume) returns to its resting size.
The thorax is the chest. The thorax is the portion of the trunk consisting of the
sternum, the costal cartilages, the ribs, and the bodies of the thoracic vertebrae.
Located above the diaphragm, the thoracic cage is roughly cone-shaped, the narrow
portion being superior and the broad portion inferior. It is flattened from front to back.
The thoracic cage is open to the outside by way of the neck and head. This bony cage
encloses and protects the lungs and other structures of the chest cavity. The thorax
also provides support for the bones of the shoulder girdle and upper extremities. Since
the wall of the thorax is reinforced by special muscles, bones, and cartilages, we can
consider the thorax to be a "solid-walled container."