RESPIRATION 



510 



vessels (aort, az. v, pt. cav} ; its middle part is wide, and 

 encloses the heart (/. vent, r. vent), the mediastinum here 

 fusing with the visceral layer of the pericardium (vis. per) 

 and thus obliterating the space ; below this it again 

 narrows to form the ventral mediastinal space (v. med), 

 in which the thymus (p. 507) is situated. 



cent 



FIG. 138. Diagrammatic transverse section of the Rabbit's thorax in the region of 

 the ventricles, to show the relations of the pleurae and mediastinum (dotted line), 

 etc. The lungs are contracted, (x ) 



aort. dorsal aorta ; az. v. azygos vein ; cent, centrum of thoracic vertebra ; /. Ing. 

 left lung ; /. pi. left pleural cavity ; /. vent, left ventricle ; my. spinal cord ; oes. 

 gullet ; par. per. parietal layer of pericardium ; pt. cav. postcaval, close to 

 its entrance into right auricle ; r. Ing. right lung ; r. pi. right pleural cavity ; 

 r. vent, right ventricle ; st. sternum ; vis. per. visceral layer of pericardium ; 

 v. med. ventral mediastinal space. (From Parker and Haswell's Zoology.) 



In the entire animal, the air-tight pleural cavities are 

 completely filled by the lungs, so that the parietal and 

 visceral layers, of the pleurae are practically in contact, 

 there being only a lubricating serous fluid (lymph) 

 between them. The pressure of the air in the bronchial 

 cavities of the lungs is therefore sufficient to keep them 

 distended ; but as soon as the pleural cavities are 



