12 



THE HUMAN MECHANISM 



tubes, or bronchi, which pass through the mediastinum out- 

 ward, one to the right lung, the other to the left. The pleural 

 lining of the mediastinum is pushed outward by these tubes 

 and, as they end in the lungs, forms the pleural covering of 

 the latter (Fig. 5). Consequently the organs of the pleural 

 cavity either lie within the mediastinum (heart, oesophagus, 



FIG. 4. Cross section of chest posterior to branching of trachea 



A, bronchus, entering the lung ; B, the aorta cut at its origin and again at the 



descending part of its arch; C, the pericardial space; D, the pleural cavity; 



P. A., the pulmonary artery 



trachea, etc.) or else are covered by extensions of the 

 mediastinal pleura (bronchi and lungs). 



The abdominal cavity is not similarly separated into right 

 and left halves ; but a membrane, the mesentery, passes ven- 

 trally from the dorsal wall to the stomach and intestine, 

 which are slung in it somewhat as a man lies in a hammock. 

 The line of attachment of this mesentery to the small intes- 

 tine is much longer than that of its attachment to the body 

 wall ; hence it has the general shape of a ruffle, or flounce 

 an arrangement which permits the suspension of the very 



