CELLS AND THEIR ASSOCIATION 51 



requires great force to pull them apart until air begins 

 to penetrate between them. 



The thorax is divided vertically into right and left 

 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 

 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 pleura and called the peritoneum. The principal 

 organs contained in the abdomen are those belonging 

 to the digestive system. The central feature of this 

 system is the alimentary canal which, as a matter of fact, 

 is not strictly confined to the abdominal cavity since it 

 begins at the mouth, extends through the thorax, and 

 at its lower termination traverses the pelvis to open at 

 the anus. Nevertheless, the great part of the digestive 

 tract is in the abdomen. The liver and the pancreas 

 are appended to the canal and the spleen is associated 

 with it though perhaps less directly. The kidneys are 

 exposed to view when the organs of digestion are re- 

 moved from the abdominal cavity; they lie behind the 

 peritoneum and are best thought of as belonging to the 

 back rather than to the abdomen. 



In the pelvis there are found the terminal portion of 

 the alimentary canal, as already noted, the urinary 

 bladder, and the reproductive organs. The peritoneum 

 intervenes between the pelvic cavity and that of the 

 abdomen above. Details will be added as we discuss 

 the particular organs which have merely been mentioned 

 at this place. 



Anatomical Terms. It will be well to define here a 

 few terms of anatomy which must often be used. Right 

 and left have their ordinary meaning. Dorsal signifies 

 toward the back and ventral is its opposite (venter, the 



