Through which the air of the lungs enters and passes out,
consists of from sixteen to twenty-six cartilaginous rings,
posteriorly incomplete, lying horizontally one above the
other.
These rings are connected by a membrane covering them externally
and internally. As they enter the cavity of the chest, they divide
into two branches, likewise composed of rings, one entering the
right, the other the left lung. Before they join the lungs they
divide again into several smaller branches, which again subdivide
fork-like in the lungs, and terminate in numberless little
grape-like clusters of hollow vesicles. The diameter of the
trachea in adults is from one-half to three-fourths of an
inch when at rest.