_A_, Aorta; _Br._, bronchi; _P.A._, pulmonary arteries; _l_, lymph
gland; _R.A._, right atrium; _L.A._; left atrium; _R.V._, right
ventricle; _L.V._, left ventricle of heart.
]
The right pleura forms a special =sagittal fold= (Plica venæ cavæ) about
a handbreadth to the right of the median plane, which incloses the
posterior vena cava in its upper edge and gives off a small accessory
fold for the right phrenic nerve. The fold arises from the thoracic
floor and from the diaphragm below the foramen venæ cavæ, and intervenes
between the mediastinal lobe and the body of the right lung. It is
delicate and lace-like.
The posterior mediastinum is very delicate below the œsophagus, and
usually appears fenestrated; when these apertures are present, the two
pleural cavities communicate with each other.[127]
The pleural sacs contain a serous fluid, the =liquor pleuræ=; in health
there is only a sufficient amount to moisten the surface, but it
accumulates rapidly after death.
[Illustration:
FIG. 360.—CROSS-SECTION OF THORAX OF NEW-BORN FOAL. Ribs are numbered.
]