THE VISCERA. 



I. THE BODY CAVITY. 



The greater part of the viscera are situated in the body 

 cavity or ccelom. 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 vertebrae, 

 the ribs, the sternum, and the diaphragm. The cranial open- 

 ing of the cavity is filled by the trachea and oesophagus 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 

 pleurae, 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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