THE VISCERA. 
I. THE BODY CAVITY. 
The greater part of the viscera are situated in the body 
cavity or cceelom. This is divided by the diaphragm into two 
parts, the thoracic cavity and the abdominal cavity. Each 
is lined by a serous membrane, in which the part covering the 
outer wall of the cavity is distinguished as the parietal layer 
from the part covering the viscera, which is known as the 
visceral layer. 
The thoracic cavity is bounded by the thoracic vertebre, 
the ribs, the sternum, and the diaphragm. The cranial open- 
ing of the cavity is filled by the trachea and cesophagus as they 
enter from the neck region. The thoracic cavity is lined by 
two thin layers of tissue, the outer one of which is the fascia 
endothoracica, while the inner is the pleura. The fascia 
endothoracica is a sheet of connective tissue which lines the 
entire inner surface of the thoracic cavity, descending from the 
dorsal median line to the heart, and passing into the fibrous 
layer of the pericardium. The pleura is a thin membrane 
covering the fascia endothoracica and corresponding to the. 
peritoneum of the abdominal cavity. It forms two sacs, the 
pleure, lining respectively the right and left halves of the 
thoracic cavity. Each of these two sacs is closed, the viscera 
being suspended within them by folds of the membrane, so that 
the cavity is everywhere separated from the viscera by a sheet 
of the pleura. That portion of the pleura which lines the 
thoracic wall is known as the parietal layer; it may be divided 
into that covering the ribs (costal pleura), and that covering 
the diaphragm. That portion which covers the viscera is the 
visceral layer, or, since it covers chiefly the lungs, it may be 
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