_The larynx_ may be compared to a box open at both ends. The several
cartilages that form it are united by ligaments. It is lined by a mucous
membrane. The posterior extremity is united to the first cartilaginous ring
of the trachea. The anterior opening is closed by the epiglottis. Just
within is a V-shaped opening that is limited laterally by the folds of the
laryngeal mucous membrane, the vocal chords.

_The trachea_ is a cylindrical tube originating at the posterior extremity
of the larynx, and terminating within the chest cavity at a point just
above the heart in the right and left bronchial tubes. It is formed by a
series of cartilaginous rings joined together at their borders by ligaments
and lined by a mucous membrane.

_The bronchial tubes_ resemble the trachea in structure. They enter the
lungs a short distance from their origin, where they subdivide into
branches and sub-branches, gradually decreasing in calibre and losing the
cartilaginous rings, ligaments and muscular layer until only the thin
mucous membrane is left. They become capillary in diameter, and finally
open into the infundibula of the air cells of the lungs.

_The lungs_ take up all of the space in the thoracic cavity not occupied by
the heart, blood-vessels and oesophagus. This cavity resembles a cone in
shape that is cut obliquely downwards and forward at its base. The base is
formed by the diaphragm which is pushed forward at its middle. It is lined
by the pleura, a serous membrane, that is inflected from the wall over the
different organs within the cavity. The median folds of the pleura divide
the cavity into right and left portions. A second method of describing the
arrangement of the pleura is to state that it forms two sacks, right and
left, that enclose the lungs. The lungs are the essential organs of
respiration. The tissue that forms them is light, will float in water, is