The organs of respiration (Apparatus respiratorius) comprise the =nasal
cavity=,[120] the =pharynx=, the =larynx=, the =trachea=, the =bronchi=,
and the =lungs=. The lungs are the central organs in which the exchange
of gases between the blood and the air takes place; the other parts of
the system are passages by which the air passes to and from the lungs.
The nasal cavity opens externally at the nostrils, and communicates
behind with the pharynx through the posterior nares (Choanæ); it
contains the peripheral part of the olfactory apparatus, which mediates
the sense of smell. The pharynx is a common passage for the air and
food—a remnant of the primitive embryonic arrangement; it has been
described as a part of the digestive tube. The larynx is a complex
valvular apparatus which regulates the volume of air passing through the
tract; it is also the chief organ of voice. The trachea, and the bronchi
formed by its bifurcation, are permanently open conducting tubes. The
thorax, the pleural sacs which it contains, and the muscles which
increase or diminish the size of the cavity are also parts of the
system. The bones, joints, and muscles of the thorax have already been
described.
For topographic reasons two ductless glands, the =thyroid= and the
=thymus=, are usually described in this section, although they are in no
sense a part of the respiratory system.