The =thoracic cavity= (Cavum thoracis) is the second in point of size of
the body cavities. In form it is somewhat like a truncated cone, much
compressed laterally in its anterior part, and with the base cut off
very obliquely.
The =dorsal wall= or =roof= is formed by the thoracic vertebræ and the
ligaments and muscles connected with them.
The =lateral walls= are formed by the ribs and the intercostal muscles.
The =ventral wall= or =floor= is formed by the sternum, the cartilages
of the sternal ribs, and the muscles in connection therewith. It is
about one-half as long as the dorsal wall.
The =posterior wall=, formed by the diaphragm, is very oblique and is
strongly convex.
The =anterior aperture= or =inlet= (Apertura thoracis cranialis) is
relatively small, and of narrow, oval form. It is bounded dorsally by
the first thoracic vertebra and laterally by the first pair of ribs. It
is occupied by the longus colli muscles, the trachea, œsophagus,
vessels, nerves, and the prepectoral lymph glands.
The cavity is lined by the =endothoracic fascia= and by the =pleuræ=.
A longitudinal =septum=, termed the =mediastinum= (Septum mediastinale),
extends from the dorsal wall to the ventral and posterior walls, and
subdivides the cavity into two lateral chambers which contain the lungs.
Each of these chambers is lined by a serous membrane called the pleura,
and is called a =pleural cavity= (Cavum pleuræ). The mediastinum is, for
the most part, not median in position; this is largely due to the fact
that the largest organ contained in it, the heart, is placed more on the
left side; consequently the right pleural cavity and lung are larger
than the left. Practically all the organs in the thorax are in the
mediastinal space with the exception of the lungs, the posterior vena
cava, and the right phrenic nerve. The part in which the heart and the
pericardium are situated, together with that above it, is usually called
the middle mediastinal space; the parts before and behind this are
termed the anterior and posterior mediastinal spaces respectively.