10 



THE HUMAN MECHANISM 



6. The body cavities. There is one striking and important 

 structural difference between the trunk and the limbs ; the 

 former contains a central body cavity, completely filled, how- 

 ever, with various organs, while the arms and legs are each 

 composed of a continuous mass of tissues, namely, muscle, 



bloodvessels, nerves, 

 bone, etc., all bound 

 together by connec- 

 tive tissue (Figs. 1 

 and 2). 



The cavity of the 

 trunk, or body cav- 

 ity, is subdivided 

 transversely by the 

 dome-shaped muscle 

 known as the dia- 

 phragm into two 

 cavities an ante- 

 rior, known as the 

 thoracic, or pleural, 

 cavity ; and a poste- 

 rior, known as the 

 abdominal, or perito- 

 neal, cavity. Both 

 cavities are lined by 

 a thin, smooth, shiny 

 membrane, that of 

 the thoracic being 

 known as the pleura, and that of the abdominal as the 

 peritoneum. 



Filling the pleural cavity are found the heart, lungs, 

 oesophagus, windpipe (or trachea), and many great blood vessels ; 

 filling the abdominal cavity, the stomach, the small intestine, 

 the large intestine, the liver, the pancreas, the kidneys, the 

 spleen, and other organs, together with numerous large and 



FIG. 2. The thoracic, or pleural, and the abdomi- 

 nal, or peritoneal, cavities filled with organs 



