RESPIRATION 



519 



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. 



i/s.per 

 par-per- 



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

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

 etc. The lungs are contracted. ( x f .) 



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

 left lung ; /. pi. left pleuraf cavity ; f. vent, left ventricle ; my. spinal cord ; o. 

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

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

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

 v. med. ventral mediastinal space. (From Parker and Harwell'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 



