CELLS AND THEIR ASSOCIATION 51 



sections by a development of connective tissue called 

 the mediastinum. In this is suspended the heart sur- 

 rounded by a sac, the pericardium. On either side of 

 the mediastinum is a typic cavity, lined with a smooth, 

 moist membrane which is in contact with another 



FIG. 6. Suggesting the thoracic and abdominal cavities parted by 

 the diaphragm. The abdominal viscera are drawn upward, creating, 

 a space above the bladder where none normally exists. 



.membrane of the same nature covering the lung. The 

 linings of these two cavities and the coverings of the 

 two lungs are spoken of as the layers of the pleura. 



The abdominal cavity is lined by a membrane similar 

 to the Dleura and called the peritoneum. The principal 

 organs contained in the abdomen are those belonging 



